Method and apparatus for automatically balancing a blower in any blown film extrusion line

ABSTRACT

The present invention is directed to a method and apparatus for startup of extruded film tubes. A controller member is utilized to provide control signals to a supply blower which supplies air to the extruded film tube in an amount corresponding to a supply control signal, and an exhaust blower which exhausts air from the extruded film tube in an amount corresponding to an exhaust control signal. The controller member executes program instructions which define at least one control routine for automatic and coordinated control of the supply blower and the exhaust blower during starting of the extruded film tube, by directing a series of supply control signals to the supply blower and exhaust control signals to the exhaust blower. When a valve is utilized to control the flow of air from the supply blower to the extruded film tube, the controller is also utilized to adjust the supply blower and the exhaust blower in order to optimize operation of the valve by placing the valve in a substantially linear operating range. Additionally, the controller member may be utilized to execute program instructions in order to implement a bubble break detection routine which utilizes software timers in combination with monitoring of a position signal in order to determine collapse or break of the extruded film tube.

BACKGROUND OF THE INVENTION CROSS REFERENCE TO RELATED APPLICATION

[0001] This patent application includes a disclosure which, in largeportion, is similar to the disclosure in U.S. patent application Ser.No. 08/658,369, entitled “Method and Apparatus for Cooling Extruded FilmTubes”, which was filed on Jun. 5, 1996. However, this application isnot a continuation or continuation-in-part of this co-pendingapplication.

[0002] 1. Field of the Invention:

[0003] This invention relates generally to blown film extrusion lines,and specifically to operations of the inlet and outlet blowers of blownfile systems.

[0004] 2. Description of the Prior Art:

[0005] Blown film extrusion lines are used to manufacture plastic bagsand plastic sheets. A molten tube of plastic is extruded from an annulardie, and then stretched and expanded to a larger diameter and a reducedradial thickness by the action of overhead nip rollers and internal airpressure. Typically, air is entrained by one or more blowers to providea cooling medium which absorbs heat from the molten material and speedsup the change in state from a molten material back to a solid material.Additionally, blowers are used to provide air pressure which is utilizedto control the size and thickness of the film tube. One type of blownfilm extrusion line utilizes an air flow on the exterior surface of thefilm tube in order to absorb heat. A different, and more modern, type ofblown film extrusion line utilizes both an external flow of cooling airand an internal flow of cooling air in order to cool and size the filmtube.

[0006] As stated above, blowers are utilized to provide air to theinterior of the film tube. Typically, a supply blower is provided inorder to supply air to the interior of the film tube, and an exhaustblower is provided in order to exhaust air from the interior of the filmtube. Typically, the supply blower and exhaust blower are underelectrical control during production operations. However, during startupof the extrusion process, in the prior art, a great deal of humanintervention is required in order to establish the bubble. Typically, ahuman operator will first control the supply blower until the extrudedfilm tube is closed at its upper end by engagement with the overhead niprollers. Then, the exhaust blowers utilized to remove air in order toprevent expansion and eventual breaking of the extruded film tube. Abalance between the supply blower and the exhaust blower must beobtained in order to allow for continuous production of the extrudedfilm tube. The startup of an extruded film is a relatively difficultoperation to perform, and generally requires a relatively highly-skilledemployee to oversee or perform the startup operations.

SUMMARY OF THE INVENTION

[0007] It is one objection of the present invention to provide a methodand apparatus for startup of an extruded film tube which includes asupply blower, an exhaust blower, and a controller member, includingexecutable program instructions which define at least one controlroutine for automatic and coordinated control of the supply blower andthe exhaust blower during startup.

[0008] The control routines may comprise a startup routine which isutilized in initiating the extruded film tube, a blower optimizationroutine which is utilized to optimize the operating speeds of either orboth of the supply blower and the exhaust blower, and a valveoptimization routine wherein an operating condition is established foreither or both of the supply blower and the exhaust blower in a mannerwhich optimizes operation of a valve member which is utilized to controlthe application of air from the supply blower to the extruded film tube.

[0009] It is yet another objective of the present invention to utilizeprior recorded operating conditions for either or both of the supplyblower and exhaust blower in order to take advantage of the value ofprior experience with a particular blown film line.

[0010] It is yet another objective of the present invention to providean additional routine which can be utilized to detect bubble breaksduring and after the startup operations.

[0011] These and other objectives are achieved as is now described.

[0012] A method and apparatus is provided for startup of an extrudedfilm tube. The method and apparatus is used in a blown film extrusionapparatus in which film is extruded as a tube from an annular die andpulled along a predetermined path. A means is provided for varying aquantity of air within the extruded film tube. Preferably, the meansincludes a supply blower which supplies air to the extruded film tube inan amount corresponding to a supply control signal, and an exhaustblower which exhausts air from the extruded film tube in an amountcorresponding to an exhaust control signal. A controller member isprovided. The controller member includes executable program instructionswhich define at least one control routine for automatic and coordinatedcontrol of the means for varying during startup of the extruded filmtube. The controller directs a series of supply control signals to thesupply blower and exhaust control signals to the exhaust blower in orderto set their optimum operating conditions. In the preferred embodiment,a control interface is provided for receiving operator instructionsduring startup of the extruded film tube. The controller furtherincludes program instructions for receiving the operating instructionsand integrating the operating instructions into the at least one controlroutine. In the preferred embodiment, a valve member is provided betweenthe supply blower and the extruded film tube. The valve member is undercontrol of the controller member, and is utilized for varying admissionof air into the extruded film tube and for controlling the circumferenceof the extruded film tube after startup of the extruded film tube.

[0013] In the preferred embodiment, a variety of control routines may beprovided. In a startup routine, the controller member initiatesoperation of the supply blower and the exhaust blower by firstinitiating operation of the supply blower in accordance with at leastone predetermined operating parameter, and then initiating the exhaustblower in accordance with at least one predetermined parameter.

[0014] In a blower optimization routine, at least one of the supplycontrol signal and the exhaust control signal is determined, at least inpart, from at least one prior recorded control signal. Preferably, atable is generated in controller memory which records over time theoptimum settings of the supply blower and exhaust blower. Duringstartup, the blower optimization routine may be utilized to takeadvantage of the prior historical knowledge of the blown film apparatus.

[0015] In a valve optimization routine, an operating condition isestablished for at least one of the supply blower and the exhaust blowerin a manner which optimizes operation of the valve member. In thepreferred embodiment of the present invention, the objective is to allowthe valve member to operate in a preferred and substantially linearrange of closure conditions.

[0016] In a bubble break detection routine, a position signal (whichindicates the position or size of the bubble) is utilized in combinationwith at least one software timer in order to detect a break in theextruded film tube. In the preferred embodiment of the presentinvention, one software timer is utilized to suppress operation of thebubble break detection routine until a portion of the startup routine iscompleted. Then, a second software timer is utilized in order toidentify unacceptably long intervals of interruption in the positionsignal, which is interpreted to identify a break or collapse of theextruded film tube.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The novel features believed characteristic of the invention areset forth in the appended claims. The invention itself however, as wellas a 5 preferred mode of use, further objects and advantages thereof,will best be understood by reference to the following detaileddescription of an illustrative embodiment when read in conjunction withthe accompanying drawings, wherein:

[0018]FIG. 1 is a view of a blown film extrusion line equipped with theimproved control system of the present invention;

[0019]FIG. 2 is a view of the die, sizing cage, control subassembly androtating frame of the blown film tower of FIG. 1;

[0020]FIG. 3 is a view of the acoustic transducer of the improvedcontrol system of the present invention coupled to the sizing cage ofthe blown film extrusion line tower adjacent the extruded film tube ofFIGS. 1 and 2;

[0021]FIG. 4 is a view of the acoustic transducer of FIG. 3 coupled tothe sizing cage of the blown film tower, in two positions, one positionbeing shown in phantom;

[0022]FIG. 5 is a schematic and block diagram view of the preferredcontrol system of the present invention;

[0023]FIG. 6 is a schematic and block diagram view of the preferredcontrol system of FIG. 5, with special emphasis on the supervisorycontrol unit;

[0024]FIG. 7A is a schematic and block diagram view of the signalsgenerated by the ultrasonic sensor which pertain to the position of theblown film layer;

[0025]FIG. 7B is a view of the ultrasonic sensor of FIG. 3 coupled tothe sizing cage of the blown film tower, with permissible extruded filmtube operating ranges indicated thereon;

[0026]FIG. 8A is a flow chart of the preferred filtering process appliedto the current position signal generated by the acoustic transducer;

[0027]FIG. 8B is a graphic depiction of the operation of the filteringsystem;

[0028]FIG. 9 is a schematic representation of the automatic sizing andrecovery logic (ASRL) of FIG. 6;

[0029]FIG. 10 is a schematic representation of the health/state logic(HSL) of FIG. 6;

[0030]FIG. 11 is a schematic representation of the loop mode controllogic (LMCL) of FIG. 6;

[0031]FIG. 12 is a schematic representation of the volume setpointcontrol logic (VSCL) of FIG. 6;

[0032]FIG. 13 is a flow chart representation of the output clamp of FIG.6.

[0033]FIG. 14 is a schematic and block diagram, and flowchart views ofthe preferred alternative emergency condition control system of thepresent invention, which provides enhanced control capabilities fordetected overblown and underblown conditions, as well as when thecontrol system determines that the extruded film tube has passed out ofrange of the sensing transducer;

[0034]FIG. 15 is a schematic and block diagram view of the signalsgenerated by the ultrasonic sensor which pertain to the position of theblown film layer;

[0035]FIG. 16 is a view of the ultrasonic sensor of FIG. 3 coupled tothe sizing cage of the blown film tower, with permissible extruded filmtube operating ranges indicated thereon;

[0036]FIG. 17 is a schematic representation of the automatic sizing andrecovery logic (ASRL) of FIG. 14;

[0037]FIG. 18 is a schematic representation of the health/state logic(HSL) of FIG. 14;

[0038]FIG. 19 is a schematic representation of the loop mode controllogic (LMCL) of FIG. 14;

[0039]FIG. 20 is a schematic representation of the volume setpointcontrol logic (VSCL) of FIG. 14;

[0040]FIG. 21 is a flow chart representation of the output clamp of FIG.14;

[0041]FIG. 22 is a schematic and block diagram view of emergencycondition control logic block of FIG. 14;

[0042]FIGS. 23A through 23G depict the preferred software routinesutilized in the present invention, including a first filter routinewhich is utilized during relatively unstable intervals of operation, anda second dynamic filtering routine which is utilized during relativelystable intervals of operation;

[0043]FIG. 24 is a graphic depiction of the normal operation of thefiltering system;

[0044]FIG. 25A is a graph which depicts the emergency condition controlmode of operation response to the detection of an underblown condition,with the X-axis representing time and the Y-axis representing positionof the extruded film tube;

[0045]FIG. 25B is a graph of the binary condition of selected operatingblocks of the block diagram depiction of FIG. 22, and can be read incombination with FIG. 25A, wherein the X-axis represents time, and theY-axis represents the binary condition of selected operational blocks;

[0046]FIG. 26A is a graph which depicts the emergency condition controlmode of operation response to the detection of an underblown condition,with the X-axis representing time and the Y-axis representing positionof the extruded film tube;

[0047]FIG. 26B is a graph of the binary condition of selected operatingblocks of the block diagram depiction of FIG. 22, and can be read incombination with FIG. 26A, wherein the X-axis represents time, and theY-axis represents the binary condition of selected operational blocks;

[0048]FIG. 27A is a graph which depicts the emergency condition controlmode of operation response to the detection of an underblown condition,with the X-axis representing time and the Y-axis representing positionof the extruded film tube;

[0049]FIG. 27B is a graph of the binary condition of selected operatingblocks of the block diagram depiction of FIG. 22, and can be read incombination with FIG. 27A, wherein the X-axis represents time, and theY-axis represents the binary condition of selected operational blocks;

[0050]FIG. 28 is a schematic and block diagram depiction of oneembodiment of the improved air flow control system of the presentinvention;

[0051]FIG. 29 is a simplified and partial fragmentary and longitudinalsection view of the preferred air flow control device used with the airflow control system of the present invention;

[0052]FIG. 30 is a schematic depiction of a IBC blown film extrusionline equipped with mass air flow sensors in communication with both asupply of cooling air and an exhaust of cooling air, which may beutilized to obtain uniformity in the mass air flow of the cooling airstream supply to the interior of the blown film tube;

[0053]FIG. 31 is a schematic depiction of an IBC blown film lineequipped with mass air flow sensors for controlling the supply andexhaust of air to the interior of the blown film tube, and additionallyequipped with a mass air flow sensor for monitoring and controlling thesupply of external cooling air;

[0054]FIGS. 32, 33, 34, and 35 are schematic depictions of an externalcooling air system for a blown film extrusion line, with a mass air flowsensor provided to allow control over an adjustable air flow attributemodifier;

[0055]FIG. 36 is a flowchart representation of computer programimplemented operations for achieving a feedback control loop for a blownfilm system;

[0056]FIG. 37A is a schematic representation of the prior art control ofsupply and exhaust blowers;

[0057]FIG. 37B and FIG. 37C are graphical representations of theperformance curves for supply and exhaust blowers;

[0058]FIG. 37D is a block diagram and schematic representation of thestartup control apparatus of the present invention;

[0059]FIG. 37E is a flowchart representation of the control routine ofthe startup control apparatus of the present invention;

[0060]FIG. 37F(1)-37F(2) is a flowchart representation of the startupmode of operation of FIG. 37E;

[0061]FIGS. 37G through 37J are flowchart representations of the runmode of FIG. 37E;

[0062]FIG. 37K is a flowchart representation of the balance mode of FIG.37E;

[0063]FIG. 37L is a pictorial representation of an array of recordedprior control settings for the supply and exhaust blowers; and

[0064]FIG. 37M is a flowchart representation of a bubble break detectionroutine.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

[0065] In this detailed description of the invention, FIGS. 1 through29, and accompanying text, provide a very detailed overview of aninternal-bubble-cooling blown film extrusion system which is equippedwith a preferred sizing control system. FIGS. 30 through 36, andaccompanying text, provide a description of the preferred method andapparatus for cooling extruded film tubes of the present invention usedeither in combination with the preferred sizing control apparatus, oralone.

[0066]FIG. 1 is a view of blown film extrusion line 11, which includes anumber of subassemblies which cooperate to produce plastic bags and thelike from plastic resin. The main components include blown film tower13, which provides a rigid structure for mounting and aligning thevarious subassemblies, extruder subassembly 15, die subassembly 17,blower subassembly 19, stack 21, sizing cage 23, collapsible frame25,nips 27, control subassembly 28 and rollers 29.

[0067] Plastic granules are fed into hopper 31 of extruder subassembly15. The plastic granules are melted and fed by extruder 33 and pushedinto die subassembly 17, and specifically to annular die 37. The moltenplastic granules emerge from annular die 37 as a molten plastic tube 39,which expands from the die diameter to a desired final diameter, whichmay vary typically between two to three times the die diameter.

[0068] Blower subassembly 19 includes a variety of components whichcooperate together to provide a flow of cooling air to the interior ofmolten plastic tube 39, and also along the outer periphery of moltenplastic tube 39. Blower subassembly includes blower 41 which pulls airinto the system at intake 43, and exhausts air from the system atexhaust 45. The flow of air into molten plastic tube 39 is controlled atvalve 47. Air is also directed along the exterior of molten plastic tubefrom external air ring 49, which is concentric to annular die 37. Air issupplied to the interior of molten plastic tube 39 through internal airdiffuser 51. Air is pulled from the interior of molten plastic tube 39by exhaust stack 53.

[0069] The streams of external and internal cooling airs serve to hardenmolten plastic tube 39 a short distance from annular die 37. The line ofdemarcation between the molten plastic tube 39 and the hardened plastictube 55 is identified in the trade as the “frost line.” Normally, thefrost line is substantially at or about the location at which the moltenplastic tube 39 is expanded to the desired final diameter.

[0070] Adjustable sizing cage 23 is provided directly above annular die38 and serves to protect and guide the plastic tube 55 as it is drawnupward through collapsible frame 25 by nips 27. Afterwards, plastic tube55 is directed through a series of rollers 57, 59, 61, and 63 whichserve to guide the tube to packaging or other processing equipment.

[0071] In some systems, rotating frame 65 is provided for rotatingrelative to blown film tower 13. It is particularly useful in rotatingmechanical feeler arms of the prior art systems around plastic tube 55to distribute the deformations. Umbilical cord 67 is provided to allowelectrical conductors to be routed to rotating frame 65. Rotating frame65 rotates at bearings 71, 73 relative to stationary frame 69.

[0072] Control subassembly 28 is provided to monitor and control theextrusion process, and in particular the circumference of plastic tube55. Control subassembly 28 includes supervisory control unit, andoperator control panel 77.

[0073]FIG. 2 is a more detailed view of annular die 37, sizing cage 23,control subassembly 28, and rotating frame 65. As shown in FIG. 2,supervisory control unit 75 is electrically coupled to operator controlpanel 77, valve 47, and acoustic transducer 79. These componentscooperate to control the volume of air contained within extruded filmtube 81, and hence the thickness and diameter of the extruded film tube81. Valve 47 controls the amount of air directed by blower 41 intoextruded film tube 81 through internal air diffuser 51.

[0074] If more air is directed into extruded film tube 81 by internalair diffuser 51 than is exhausted from extruded film tube 81 by exhauststack 43, the circumference of extruded film tube 81 will be increased.Conversely, if more air is exhausted from the interior of extruded filmtube 81 by exhaust stack 53 than is inputted into extruded film tube 81by internal air diffuser 51, the circumference of extruded film tube 81will decrease.

[0075] In the preferred embodiment, valve 41 is responsive tosupervisory control unit 75 for increasing or decreasing the flow of airinto extruded film tube 81. Operator control panel 77 serves to allowthe operator to select the diameter of extruded film tube 81. Acoustictransducer 79 serves to generate a signal corresponding to thecircumference of extruded film tube 81, and direct this signal tosupervisory control unit 75 for comparison to the circumference settingselected by the operator at operator control panel 77.

[0076] If the actual circumference of extruded film tube 81 exceeds theselected circumference, supervisory control unit 75 operates valve 47 torestrict the passage of air from blower 41 into extruded film tube 81.This results in a decrease in circumference of extruded film tube 81.Conversely, if the circumference of extruded film tube 81 is less thanthe selected circumference, supervisory control unit 75 operates onvalve 47 to increase the flow of air into extruded film tube 81 andincrease its circumference. Of course, extruded film tube 81 willfluctuate in circumference, requiring constant adjustment andreadjustment of the inflow of air by operation of supervisory controlunit 75 and valve 47.

[0077]FIG. 3 is a view of ultrasonic sensor 89 of the improved controlsystem of the present invention coupled to sizing cage 23 adjacentextruded film tube 81. In the preferred embodiment, acoustic transducer79 comprises an ultrasonic measuring and control system manufactured byMassa Products Corporation of Hingham, Massachusetts, Model Nos. M-4000,M410/215, and M450, including a Massa Products ultrasonic sensor 89. Itis an ultrasonic ranging and detection device which utilizes highfrequency sound waves which are deflected off objects and detected. Inthe preferred embodiment, a pair of ultrasonic sensors 89 are used, oneto transmit sonic pulses, and another to receive sonic pulses. Forpurposes of simplifying the description only one ultrasonic sensor 89 isshown, and in fact a single ultrasonic sensor can be used, first totransmit a sonic pulse and then to receive the return in an alternatingfashion. The elapsed time between an ultrasonic pulse being transmittedand a significant echo being received corresponds to the distancebetween ultrasonic sensor 89 and the object being sensed. Of course, thedistance between the ultrasonic sensor 89 and extruded film tube 81corresponds to the circumference of extruded film tube 81. In thepresent situation, ultrasonic sensor 89 emits an interrogatingultrasonic beam 87 substantially normal to extruded film tube 81 andwhich is deflected from the outer surface of extruded film tube 81 andsensed by ultrasonic sensor 89.

[0078] The Massa Products Corporation ultrasonic measurement and controlsystem includes system electronics which utilize the duration of timebetween transmission and reception to produce a useable electricaloutput such as a voltage or current. In the preferred embodiment,ultrasonic sensor 89 is coupled to sizing cage 23 at adjustable coupling83. In the preferred embodiment, ultrasonic sensor 89 is positionedwithin seven inches of extruded film tube 81 to minimize the impact ofambient noise on a control system. Ultrasonic sensor 89 is positioned sothat interrogating ultrasonic beam 87 travels through a path which issubstantially normal to the outer surface of extruded film tube 81, tomaximize the return signal to ultrasonic sensor 89.

[0079]FIG. 4 is a view of ultrasonic sensor 89 of FIG. 3 coupled tosizing cage 23 of the blown film tower 13, in two positions, oneposition being shown in phantom. In the first position, ultrasonicsensor 89 is shown adjacent extruded film tube 81 of a selectedcircumference. When extruded film tube 81 is downsized to a tube havinga smaller circumference, ultrasonic sensor 89 will move inward andoutward relative to the central axis of the adjustable sizing cage,along with the adjustable sizing cage 23. The second position is shownin phantom with ultrasonic sensor 89 shown adjacent extruded film tube81 of a smaller circumference. For purposes of reference, internal airdiffuser 51 and exhaust stack 53 are shown in FIG. 4. The sizing cage isalso movable upward and downward, so ultrasonic sensor 89 is alsomovable upward and downward relative to the frostline of the extrudedfilm tube 81.

[0080]FIG. 5 is a schematic and block diagram view of the preferredcontrol system of the present invention. The preferred acoustictransducer 79 of the present invention includes ultrasonic sensor 89 andtemperature sensor 91 which cooperate to produce a current positionsignal which is independent of the ambient temperature. Ultrasonicsensor 89 is electrically coupled to ultrasonic electronics module 95,and temperature sensor 91 is electrically coupled to temperatureelectronics module 97. Together, ultrasonic electronics module 95 andtemperature electronics module 97 comprise transducer electronics 93.Four signals are produced by acoustic transducer 79, including oneanalog signal, and three digital signals.

[0081] As shown in FIG. 5, four conductors couple transducer electronicsto supervisory control unit 75. Specifically, conductor 99 routes a 0 to10 volts DC analog input to supervisory control unit 75. Conductors 101,103, and 105 provide digital signals to supervisory control unit 75which correspond to a target present signal, maximum override, andminimum override, These signals will be described below in greaterdetail.

[0082] Supervisory control unit 75 is electrically coupled to setpointdisplay 109 through analog display output 107. An analog signal between0 and 10 volts DC is provided to setpoint display 109 which displays theselected distance between ultrasonic sensor 89 and extruded film tube81. A distance is selected by the operator through distance selector111. Target indicator 113, preferably a light, is provided to indicatethat the target (extruded film tube 81 ) is in range. Distance selector111 is electrically coupled to supervisory control unit 75 by distancesetting conductor 119. Target indicator 113 is electrically coupled tosupervisory control unit 75 through target present conductor 121.

[0083] Supervisory control unit 75 is also coupled via valve controlconductor 123 to proportional valve 125. In the preferred embodiment,proportional valve 125 corresponds to valve 47 of FIG. 1, and is apressure control component manufactured by Proportionair ofMcCordsville, Ind., Model No. BB1. Proportional valve 125 translates ananalog DC voltage provided by supervisory control unit 75 into acorresponding pressure between 0.5 and 1.2 bar. Proportional valve 125acts on rotary valve 129 through cylinder 127. Pressurized air isprovided to proportional valve 125 from pressurized air supply 131through 20 micron filter 133.

[0084]FIG. 6 is a schematic and block diagram view of the preferredcontrol system of FIG. 5, with special emphasis on the supervisorycontrol unit 75. Extruded film tube 81 is shown in cross-section withultrasonic sensor 89 adjacent its outer wall. Ultrasonic sensor 89 emitsinterrogating pulses which are bounced off of extruded film tube andsensed by ultrasonic sensor 89. The time delay between transmission andreception of the interrogating pulse is processed by transducerelectronics 93 to produce four outputs: CURRENT POSITION signal which isprovided to supervisory control unit 75 via analog output conductor 99,digital TARGET PRESENT signal which is provided over digital output 105,a minimum override signal (MIO signal) indicative of a collapsing orundersized bubble which is provided over digital output conductor 103,and maximum override signal (MAO signal) indicative of an overblownextruded film tube 81 which is provided over a digital output conductor101.

[0085] As shown in FIG. 6, the position of extruded film tube 81relative to ultrasonic sensor 89 is analyzed and controlled withreference to a number of distance thresholds and setpoints, which areshown in greater detail in FIG. 7A. All set points and thresholdsrepresent distances from reference R. The control system of the presentinvention attempts to maintain extruded film tube 81 at a circumferencewhich places the wall of extruded film tube 81 at a tangent to the lineestablished by reference A. The distance between reference R and setpoint A may be selected by the user through distance selector 111. Thisallows the user to control the distance between ultrasonic sensor 89 andextruded film tube 81.

[0086] The operating range of acoustic transducer 79 is configurable bythe user with settings made in transducer electronics 93. In thepreferred embodiment, using the Massa Products transducer, the range ofoperation of acoustic transducer 79 is between 3 to 24 inches.Therefore, the user may select a minimum circumference threshold C and amaximum circumference threshold B, below and above which an error signalis generated. Minimum circumference threshold C may be set by the userat a distance d3 from reference R. Maximum circumference threshold B maybe selected by the user to be a distance d2 from reference R. In thepreferred embodiment, setpoint A is set a distance of 7 inches fromreference R. Minimum circumference threshold C is set a distance of10.8125 inches from reference R. Maximum circumference threshold B isset a distance of 4.1 inches from reference R. Transducer electronics 93allows the user to set or adjust these distances at will provided theyare established within the range of operation of acoustic transducer 79,which is between 3 and 24 inches.

[0087] Besides providing an analog indication of the distance betweenultrasonic sensors 89 and extruded film tube 81, transducer electronics93 also produces three digital signals which provide informationpertaining to the position of extruded film tube 81. If extruded filmtube 81 is substantially normal and within the operating range ofultrasonic sensor 89, a digital “1” is provided at digital output 105.The signal is representative of a TARGET PRESENT signal. If extrudedfilm tube 81 is not within the operating range of ultrasonic sensor 89or if a return pulse is not received due to curvature of extruded filmtube 81, TARGET PRESENT signal of digital output 105 is low. Asdiscussed above, digital output 103 is a minimum override signal MIO. Ifextruded film tube 81 is smaller in circumference than the referenceestablished by threshold C, minimum override signal MIO of digitaloutput 103 is high. Conversely, if circumference of extruded film tube81 is greater than the reference established by threshold C, the minimumoverride signal MIO is low.

[0088] Digital output 101 is for a maximum override signal MAO. Ifextruded film tube 81 is greater than the reference established bythreshold B, the maximum override signal MAO is high. Conversely, if thecircumference of extruded film tube 81 is less than the referenceestablished by threshold B, the output of maximum override signal MAO islow.

[0089] The minimum override signal MIO will stay high as long asextruded film tube 81 has a circumference less than that established bythreshold C. Likewise, the maximum override signal MAO will remain highfor as long as the circumference of extruded film tube 81 remains largerthan the reference established by threshold B.

[0090] Threshold D and threshold E are also depicted in FIG. 7A.Threshold D is established at a distance d4 from reference R. ThresholdE is established at a distance d5 from reference R. Thresholds D and Eare established by supervisory control unit 75, not by acoustictransducer 79. Threshold D represents a minimum circumference thresholdfor extruded film tube 81 which differs from that established bytransducer electronics 93. Likewise, threshold E corresponds to amaximum circumference threshold which differs from that established byacoustic transducer 79. Thresholds D and E are established in thesoftware of supervisory control unit 75, and provide a redundancy ofcontrol, and also minimize the possibility of user error, since thesethreshold are established in software, and cannot be easily changed oraccidentally changed. The coordination of all of these thresholds willbe discussed in greater detail below. In the preferred embodiment,threshold C is established at 10.8125 inches from reference R. ThresholdE is established at 3.6 inches from reference R.

[0091]FIG. 7B is a side view of the ultrasonic sensor 89 coupled tosizing cage 23 of the blown film tower 13, with permissible extrudedfilm tube 81 operating ranges indicated thereon. Setpoint A is thedesired distance between ultrasonic sensor 89 and extruded film tube 81.Thresholds D and C are established at selected distances inward fromultrasonic sensor 89, and represent minimum circumference thresholds forextruded film tube 81. Thresholds B and E are established at selecteddistances from setpoint A, and establish separate maximum circumferencethresholds for extruded film tube 81. As shown in FIG. 7B, extruded filmtube 81 is not at setpoint A. Therefore, additional air must be suppliedto the interior of extruded film tube 81 to expand the extruded filmtube 81 to the desired circumference established by setpoint A.

[0092] If extruded film tube 81 were to collapse, two separate alarmconditions would be registered. One alarm condition will be establishedwhen extruded film tube 81 falls below threshold C. A second andseparate alarm condition will be established when extruded film tube 81falls below threshold D. Extruded film tube 81 may also becomeoverblown. In an overblown condition, two separate alarm conditions arepossible. When extruded film tube 81 expands beyond threshold B, analarm condition is registered. When extruded film tube 81 expandsfurther to extend beyond threshold E, a separate alarm condition isregistered.

[0093] As discussed above, thresholds C and B are subject to useradjustment through settings in transducer electronics 93. In contrast,thresholds D and E are set in computer code of supervisory control unit75, and are not easily adjusted. This redundancy in control guardsagainst accidental or intentional missetting of the threshold conditionsat transducer electronics 93. The system also guards against thepossibility of equipment failure in transducer 79, or gradual drift inthe threshold settings due to deterioration, or overheating of theelectronic components contained in transducer electronics 93.

[0094] Returning now to FIG. 6, operator control panel 137 andsupervisory control unit 75 will be described in greater detail.Operator control panel 137 includes setpoint display 109, which servesto display the distance d1 between reference R and setpoint A. Setpointdisplay 109 includes a 7 segment display. Distance selector 111 is usedto adjust setpoint A. Holding the switch to the “+” position increasesthe circumference of extruded film tube 81 by decreasing distance d1between setpoint A and reference R. Holding the switch to the “−”position decreases the diameter of extruded film tube 81 by increasingthe distance between reference R and setpoint A.

[0095] Target indicator 113 is a target light which displays informationpertaining to whether extruded film tube 81 is within range ofultrasonic transducer 89, whether an echo is received at ultrasonictransducer 89, and whether any alarm condition has occurred. Blowerswitch 139 is also provided in operator control panel 137 to allow theoperator to selectively disconnect the blower from the control unit. Asshown in FIG. 6, all these components of operator control panel 137 areelectrically coupled to supervisory control unit 75.

[0096] Supervisory control unit 75 responds to the information providedby acoustic transducer 79, and operator control panel 137 to actuateproportional valve 125. Proportional valve 125 in turn acts uponpneumatic cylinder 127 to rotate rotary valve 129 to control the airflow to the interior of extruded film tube 81.

[0097] With the exception of analog to digital converter 141, digital toanalog converter 143, and digital to analog converter 145 (which arehardware items), supervisory control unit 75 is a graphic representationof computer software resident in memory of supervisory control unit 75.In the preferred embodiment, supervisory control unit 75 comprises anindustrial controller, preferably a Texas Instrument brand industrialcontroller Model No. PM550. Therefore, supervisory control unit 75 isessentially a relatively low-powered computer which is dedicated to aparticular piece of machinery for monitoring and controlling. In thepreferred embodiment, supervisory control unit 75 serves to monitor manyother operations of blown film extrusion line 11. The gauging andcontrol of the circumference of extruded film tube 81 through computersoftware is one additional function which is “piggybacked” onto theindustrial controller. Alternately, it is possible to provide anindustrial controller or microcomputer which is dedicated to themonitoring and control of the extruded film tube 81. Of course,dedicating a microprocessor to this task is a rather expensivealternative.

[0098] For purposes of clarity and simplification of description, theoperation of the computer program in supervisory control unit 75 havebeen segregated into operational blocks, and presented as anamalgamation of digital hardware blocks. In the preferred embodiment,these software subcomponents include: software filter 149, health statelogic 151, automatic sizing and recovery logic 153, loop mode controllogic 155, volume setpoint control logic 157, and output clamp 159.These software modules interface with one another, and to PI loopprogram 147 of supervisory control unit 75. PI loop program is asoftware routine provided in the Texas Instruments' PM550 system. Theproportional controller regulates a process by manipulating a controlelement through the feedback of a controlled output. The equation forthe output of a PI controller is:

m=K*e+K/T∫e dt+ms

[0099] In this equation:

[0100] m=controller output

[0101] K=controller gain

[0102] e=error

[0103] T=reset time

[0104] dt=differential time

[0105] ms=constant

[0106] ∫e dt=integration of all previous errors

[0107] When an error exists, it is summed (integrated) with all theprevious errors, thereby increasing or decreasing the output of the PIcontroller (depending upon whether the error is positive or negative).Thus as the error term accumulates in the integral term, the outputchanges so as to eliminate the error.

[0108] CURRENT POSITION signal is provided by acoustic transducer 79 viaanalog output 99 to analog to digital converter 141, where the analogCURRENT POSITION signal is digitized. The digitized CURRENT POSITIONsignal is routed through software filter 149, and then to PI loopprogram 147. If the circumference of extruded film tube 81 needs to beadjusted, PI loop program 147 acts through output clamp 159 uponproportional valve 125 to adjust the quantity of air provided to theinterior of extruded film tube 81.

[0109]FIG. 8A is a flowchart of the preferred filtering process appliedto CURRENT POSITION signal generated by the acoustic transducer. Thedigitized CURRENT POSITION signal is provided from analog to digitalconverter 141 to software filter 149. The program reads the CURRENTPOSITION signal in step 161. Then, the software filter 149 sets SAMPLE(N) to the position signal.

[0110] In step 165, the absolute value of the difference between CURRENTPOSITION (SAMPLE (N)) and the previous sample (SAMPLE (N-1)) is comparedto a first threshold. If the absolute value of the difference betweenthe current sample and the previous sample is less than first thresholdT1, the value of SAMPLE (N) is set to CFS, the current filtered sample,in step 167. If the absolute value of the difference between the currentsample and the previous sample exceeds first threshold T1, in step 169,the CURRENT POSITION signal is disregarded, and the previous positionsignal SAMPLE (N-1) is substituted in its place.

[0111] Then, in step 171, the suggested change SC is calculated, bydetermining the difference between the current filtered sample CFS andthe best position estimate BPE. In step 173, the suggested change SCwhich was calculated in step 171 is compared to positive T2, which isthe maximum limit on the rate of change. If the suggested change iswithin the maximum limit allowed, in step 177, allowed change AC is setto the suggested change SC value. If, however, in step 173, thesuggested change exceeds the maximum limit allowed on the rate ofchange, in step 175, the allowed change is set to +LT2, a default valuefor allowed change.

[0112] In step 179, the suggested change SC is compared to the negativelimit for allowable rates of change, negative T2. If the suggestedchange SC is greater than the maximum limit on negative change, in step181, allowed change AC is set to negative −LT2, a default value fornegative change. However, if in step 179 it is determined that suggestedchange SC is within the maximum limit allowed on negative change, instep 183, the allowed change AC is added to the current best positionestimate BPE, in step 183. Finally, in step 185, the newly calculatedbest position estimate BPE is written to the PI loop program.

[0113] Software filter 149 is a two stage filter which first screens theCURRENT POSITION signal by comparing the amount of change, eitherpositive or negative, to threshold T1. If the CURRENT POSITION signal,as compared to the preceding position signal exceeds the threshold ofT1, the current position signal is discarded, and the previous positionsignal (SAMPLE (N-1)) is used instead. At the end of the first stage, instep 171, a suggested change SC value is derived by subtracting the bestposition estimate BPE from the current filtered sample CFS.

[0114] In the second stage of filtering, the suggested change SC valueis compared to positive and negative change thresholds (in steps 173 and179 ). If the positive or negative change thresholds are violated, theallowable change is set to a preselected value, either +LT2, or −LT2. Ofcourse, if the suggested change SC is within the limits set by positiveT2 and negative T2, then the allowable change AC is set to the suggestedchange SC.

[0115] The operation of software filter 149 may also be understood withreference to FIG. 8B. In the graph of FIG. 8B, the y-axis represents thesignal level, and the x-axis represents time. The signal as sensed byacoustic transducer 79 is designated as input, and shown in the solidline. The operation of the first stage of the software filter 149 isdepicted by the current filtered sample CFS, which is shown in the graphby cross-marks. As shown, the current filtered sample CFS operates toignore large positive or negative changes in the position signal, andwill only change when the position signal seems to have stabilized for ashort interval. Therefore, when changes occur in the current filteredsample CFS, they occur in a plateau-like manner.

[0116] In stage two of the software filter 149, the current filteredsample CFS is compared to the best position estimate BPE, to derive asuggested change SC value. The suggested SC is then compared to positiveand negative thresholds to calculate an allowable change AC which isthen added to the best position estimate BPE. FIG. 8B shows that thebest position estimate BPE signal only gradually changes in response toan upward drift in the POSITION SIGNAL. The software filtering system149 of the present invention renders the control apparatus relativelyunaffected by random noise, but capable of tracking the more “gradual”changes in bubble position.

[0117] Experimentation has revealed that the software filtering systemof the present invention operates best when the position of extrudedfilm tube 81 is sampled between 20 to 30 times per second. At thissampling rate, one is less likely to incorrectly identify noise as achange in circumference of extruded film tube 81. The preferred samplingrate accounts for the common noise signals encountered in blown filmextrusion liner.

[0118] Optional thresholds have also been derived throughexperimentation. In the first stage of filtering, threshold T1 isestablished as roughly one percent of the operating range of acoustictransducer 79, which in the preferred embodiment is twenty-one meters(24 inches less 3 inches). In the second stage of filter, thresholds+LT2 and −LT2 are established as roughly 0.30% of the operating range ofacoustic transducer 79.

[0119]FIG. 9 is a schematic representation of the automatic sizing andrecovery logic ASRL of supervisory control unit 75. As stated above,this figure is a hardware representation of a software routine. ASRL 153is provided to accommodate the many momentary false indications ofmaximum and minimum circumference violations which may be registered dueto noise, such as the noise created due to air flow between acoustictransducer 79 and extruded film tube 81. The input from maximum alarmoverride MAO is “ored” with high alarm D, from the PI loop program, at“or” operator 191. High alarm D is the signal generated by the programin supervisory control unit 75 when the circumference of extruded filmtube 81 exceeds threshold D of FIG. 7A. If a maximum override MAO signalexists, or if a high alarm condition D exists, the output of “or”operator 191 goes high, and actuates delay timer 193.

[0120] Likewise, minimum override MIO signal is “ored” at “or” operator195 with low alarm E. If a minimum override signal is present, or if alow alarm condition E exists, the output of “or” operator 195 goes high,and is directed to delay timer 197. Delay timers 193, 197 are providedto prevent an alarm condition unless the condition is held for 800milliseconds continuously. Every time the input of delay timers 193, 197goes low, the timer resets and starts from 0. This mechanism eliminatesmany false alarms.

[0121] If an alarm condition is held for 800 milliseconds continuously,an OVERBLOWN or UNDERBLOWN signal is generated, and directed to thehealth state logic 151. Detected overblown or underblown conditions are“ored” at “or” operator 199 to provide a REQUEST MANUAL MODE signalwhich is directed to loop mode control logic 156.

[0122]FIG. 10 is a schematic representation of the health-state logic151 of FIG. 6. The purpose of this logic is to control the targetindicator 113 of operator control panel 137. When in non-erroroperation, the target indicator 113 is on if the blower is on, and theTARGET PRESENT signal from digital output 105 is high. When an error issensed in the maximum override MAO or minimum override MIO lines, thetarget indicator 113 will flash on and off in one half second intervals.

[0123] In health-state logic HSL 151, the maximum override signal MAO isinverted at inverter 205. Likewise, the minimum override signal isinverted at inverter 207. “And” operator 209 serves to “and” theinverted maximum override signal MAO, with the OVERBLOWN signal, andhigh alarm signal D. A high output from “and” operator 209 indicatesthat something is wrong with the calibration of acoustic transducer 79.

[0124] Likewise, “and” operator 213 serves to “and” the inverted minimumoverride signal MIO, with the OVERBLOWN signal, and low alarm signal E.If the output of “and” operator 213 is high, something is wrong with thecalibration of acoustic transducer 79. The outputs from “and” operators209, 213 are combined in “or” operator 215 to indicate an error witheither the maximum or minimum override detection systems. The output of“or” operator 215 is channeled through oscillator 219, and inverted atinverter 217. “And” operator 211 serves to “and” the TARGET PRESENTsignal, blower signal, and inverted error signal from “or” operator 215.The output of “and” operator of 211 is connected to target indicator113.

[0125] If acoustic transducer 79 is properly calibrated, the target iswithin range and normal to the sonic pulses, and the blower is on,target indicator 113 will be on. If the target is within range andnormal to the sonic pulses, the blower is on, but acoustic transducer 79is out of calibration, target indicator 113 will be on, but will beblinking. The blinking signal indicates that acoustic transducer 79, andin particular transducer electronics 93, must be recalibrated.

[0126]FIG. 11 is a schematic representation of loop mode control logicLMCL of FIG. 6. The purpose of this software module is coordinate thetransition in modes of operation. Specifically, this software modulecoordinates automatic startup of the blown film extrusion process, aswell as changes in mode between an automated “cascade” mode and a manualmode, which is the required mode of the Pi controller to enable underand overblown conditions of the extruded film tube 81 circumference. Theplurality of input signals are provided to loop mode control logic 155,including: BLOWER ON, REQUEST MANUAL MODE, PI LOOP IN CASCADE MODE,UNDERBLOWN and OVERBLOWN. Loop mode control logic LMCL 155 provides twooutput signals: MANUAL MODE, and CASCADE MODE.

[0127]FIG. 11 includes a plurality of digital logic blocks which arerepresentative of programming operations. “Or” operator 225 “ores” theinverted BLOWER ON SIGNAL to the REQUEST MANUAL MODE SIGNAL. “And”operator 227 “ands” the inverted REQUEST MANUAL MODE SIGNAL with aninverted MANUAL MODE SIGNAL, and the BLOWER ON SIGNAL. “And” operator229 “ands” the REQUEST MANUAL MODE SIGNAL to the inverted CASCADE MODESIGNAL. This prevents MANUAL MODE and CASCADE MODE from both being on atthe same time. “And” operator 231 “ands” the MANUAL MODE SIGNAL, theinverted UNDERBLOWN SIGNAL, and the OVERBLOWN SIGNAL. “And” operator 233“ands” the MANUAL MODE SIGNAL with the UNDERBLOWN SIGNAL. This causesthe overblown condition to prevail in the event a malfunction causesboth underblown and overblown conditions to be on. Inverters 235, 237,239, 241, and 243 are provided to invert the inputted output signals ofloop mode control logic 155 were needed. Software one-shot 245 isprovided for providing a momentary response to a condition. Softwareone-shot 245 includes “and” operator 247, off-delay 249, and inverter251.

[0128] The software of loop mode control logic 155 operates to ensurethat the system is never in MANUAL MODE, and CASCADE MODE at the sametime. When manual mode is requested by REQUEST MANUAL MODE, loop modecontrol logic 155 causes MANUAL MODE to go high. When manual mode is notrequested, loop mode control logic 155 operates to cause CASCADE MODE togo high. MANUAL MODE and CASCADE MODE will never be high at the sametime. Loop mode control logic 155 also serves to ensure that the systemprovides a “bumpless transfer” when mode changes occur. The term“cascade mode” is understood in the automation industries as referringto an automatic mode which will read an adjustable setpoint.

[0129] Loop mode control logic 155 will also allow for automatic startupof the blown film extrusion process. At startup, UNDERBLOWN SIGNAL ishigh, PI LOOP IN CASCADE MODE is low, BLOWER ON SIGNAL is high. Theseinputs (and inverted inputs) are combined at “and” operators 231, 233.At startup, “and” operator 233 actuates logic block 253 to move themaximum air flow value address to the PI loop step 261. At startup, theMANUAL MODE SIGNAL is high. For the PI loop controller of the preferredembodiment, when MANUAL MODE is high, the value contained in PI loopoutput address is automatically applied to proportional valve 125. Thisresults in actuation of proportional valve 125 to allow maximum air flowto start the extruded film tube 81.

[0130] When extruded film tube 81 extends in size beyond the minimumthreshold (C and D of FIG. 7A), the UNDERBLOWN SIGNAL goes low, and thePI LOOP IN CASCADE MODE signal goes high. This causes software one-shot245 to trigger, causing logic blocks 265, 267 to push an initial biasvalue contained in a program address onto the PI loop. Simultaneously,logic blocks 269, 271 operate to place the selected setpoint value Aonto volume-setpoint control logic VSCL 157. Thereafter, volume-setpointcontrol logic VSCL 157 alone serves to communicate changes in setpointvalue A to PI loop program 147.

[0131] If an overblown or underblown condition is detected for asufficiently long period of time, the controller will request a manualmode by causing REQUEST MANUAL MODE SIGNAL to go high. If REQUEST MANUALMODE goes high, loop mode control logic LMCL 155 supervises the transferthrough operation of the logic blocks.

[0132] Loop mode control logic LMCL 155 also serves to detectedoverblown and underblown conditions. If an overblown or underblowncondition is detected by the control system, REQUEST MANUAL MODE goeshigh, and the appropriate OVERBLOWN or UNDERBLOWN signal goes high. Thelogic operators of loop mode control logic LMCL 155 operate to overridethe normal operation of the control system, and cause maximum or minimumair flow by putting the maximum air flow address 261 or minimum air flowaddress 263 to the PI output address. As stated above, when MANUAL MODEis high, these maximum or minimum air flow address values are outputteddirectly to proportional valve 125. Thus, when the extruded film tube 81is overblown, loop mode control logic LMCL 155 operates to immediatelycause proportional valve 125 to minimize air flow to extruded film tube81. Conversely, if an underblown condition is detected, loop modecontrol logic LMCL 155 causes proportional valve 125 to immediatelymaximize air flow to extruded film tube 81.

[0133]FIG. 12 depicts the operation of volume-setpoint control logicVSCL 157.

[0134] Volume setpoint control logic VSCL 157 operates to increase ordecrease setpoint A in response to changes made by the operator atdistance selector 111 of operator control panel 137, when the PI loopprogram 147 is in cascade mode, i.e. when PI LOOP IN CASCADE MODE signalis high. The INCREASE SETPOINT, DECREASE SETPOINT, and PI LOOP INCASCADE MODE signals are logically combined at “and” operators 283, and287. These “and” operators act on logic blocks 285, 289 to increase ordecrease the setpoint contained in remote setpoint address 291. When thesetpoint is either increased or decreased, logic block 293 operates toadd the offset to the remote setpoint for display, and forwards theinformation to digital to analog converter 143, for display at setpointdisplay 109 of operator control panel 137. The revised remote setpointaddress is then read by the PI loop program 147.

[0135]FIG. 13 is a flowchart drawing of output clamp 159. The purpose ofthis software routine is to make sure that the PI loop program 147 doesnot over drive the rotary valve 129 past a usable limit. Rotary valve129 operates by moving a vane to selectively occlude stationaryopenings. If the moving vane is over driven, the rotary valve will beginto open when the PI loop calls for complete closure. In step 301, theoutput of the PI loop program 147 is read. In step 303, the output of PIloop is compared to a maximum output. If it exceeds the maximum output,the PI output is set to a predetermined maximum output in step 305. Ifthe output of PI loop does not exceed the maximum output, in step 307,the clamped PI output is written to the proportional valve 125 throughdigital to analog converter 145.

[0136] FIGS. 14, through 27 will be used to describe an alternativeemergency condition control mode of operation which provides enhancedcontrol capabilities, especially when an overblown or underblowncondition is detected by the control system, or when the systemindicates that the extruded film tube is out of range of theposition-sensing transducer. In this alternative emergency conditioncontrol mode of operation, the valve of the estimated position isadvanced to a preselected valve and a more rapid change in the estimatedposition signal is allowed than during previously discussed operatingconditions, and is particularly useful when an overblown or underblowncondition is detected. In the event the control system indicates thatthe extruded film tube is out of range of the sensing transducer, theimproved control system supplies an estimated position which, in mostsituations, is a realistic estimation of the position of the extrudedfilm tube relative to the sensing transducer, thus preventing falseindications of the extruded film tube being out of range of the sensingtransducer from adversely affecting the estimated position of theextruded film tube, greatly enhancing operation of the control system.In the event an overblown condition is detected, the improved controlsystem supplies an estimated position which corresponds to the distanceboundary established for detecting an overflow condition. In the eventan underblown condition is detected, the improved control systemsupplies an estimated position which corresponds to the distanceboundary established for detecting an underblown condition.

[0137] FIGS. 14, through 27 are a block diagram, schematic, andflowchart representation of the preferred embodiment of a control systemwhich is equipped with the alternative emergency condition control modeof operation. FIGS. 25, 26, and 27 provide graphic examples of theoperation of this alternative emergency condition control mode ofoperation.

[0138]FIG. 14 is a schematic and block diagram view of the preferredalternative control system 400 of the present invention of FIG. 5, withspecial emphasis on the supervisory control unit 75, and is identical inalmost all respects to the supervisory control unit 75 which is depictedin FIG. 6; therefore, identical referenced numerals are used to identifythe various components of alternative control system 400 of FIG. 14 asare used in the control system depicted in FIG. 6.

[0139] Extruded film tube 81 is shown in cross-section with ultrasonicsensor 89 adjacent its outer wall. Ultrasonic sensor 89 emitsinterrogating pulses which are bounced off of extruded film tube andsensed by ultrasonic sensor 89. The time delay between transmission andreception of the interrogating pulse is processed by transducerelectronics 93 to produce four outputs: CURRENT POSITION signal which isprovided to supervisory control unit 75 via analog output conductor 99,digital TARGET PRESENT signal which is provided over digital output 105,a minimum override signal (MIO signal) indicative of a collapsing orundersized bubble which is provided over digital output conductor 103,and maximum override signal (MAO signal) indicative of an overblownextruded film tube 81 which is provided over a digital output conductor101.

[0140] As shown in FIG. 14, the position of extruded film tube 81relative to ultrasonic sensor 89 is analyzed and controlled withreference to a number of distance thresholds and setpoints, which areshown in greater detail in FIG. 15. All set points and thresholdsrepresent distances from reference R. The control system of the presentinvention attempts to maintain extruded film tube 81 at a circumferencewhich places the wall of extruded film tube 81 at a tangent to the lineestablished by reference A. The distance between reference R and setpoint A may be selected by the user through distance selector 111. Thisallows the user to control the distance between ultrasonic sensor 89 andextruded film tube 81.

[0141] The operating range of acoustic transducer 79 is configurable bythe user with settings made in transducer electronics 93. In thepreferred embodiment, using the Massa Products transducer, the range ofoperation of acoustic transducer 79 is between 3 to 24 inches.Therefore, the user may select a minimum circumference threshold C and amaximum circumference threshold B, below and above which an error signalis generated. Minimum circumference threshold C may be set by the userat a distance d3 from reference R. Maximum circumference threshold B maybe selected by the user to be a distance d2 from reference R. In thepreferred embodiment, setpoint A is set a distance of 7 inches fromreference R. Minimum circumference threshold C is set a distance of10.8125 inches from reference R. Maximum circumference threshold B isset a distance of 4.1 inches from reference R. Transducer electronics 93allows the user to set or adjust these distances at will provided theyare established within the range of operation of acoustic transducer 79,which is between 3 and 24 inches.

[0142] Besides providing an analog indication of the distance betweenultrasonic sensors 89 and extruded film tube 81, transducer electronics93 also produces three digital signals which provide informationpertaining to the position of extruded film tube 81. If extruded filmtube 81 is substantially normal and within the operating range ofultrasonic sensor 89, a digital “1” is provided at digital output 105.The signal is representative of a TARGET PRESENT signal. If extrudedfilm tube 81 is not within the operating range of ultrasonic sensor 89or if a return pulse is not received due to curvature of extruded filmtube 81, TARGET PRESENT signal of digital output 105 is low. Asdiscussed above, digital output 103 is a minimum override signal MIO. Ifextruded film tube 81 is smaller in circumference than the referenceestablished by threshold C, minimum override signal MIO of digitaloutput 103 is high. Conversely, if circumference of extruded film tube81 is greater than the reference established by threshold C, the minimumoverride signal MIO is low.

[0143] Digital output 101 is for a maximum override signal MAO. Ifextruded film tube 81 is greater than the reference established bythreshold B, the maximum override signal MAO is high. Conversely, if thecircumference of extruded film tube 81 is less than the referenceestablished by threshold B, the output of maximum override signal MAO islow.

[0144] The minimum override signal MIO will stay high as long asextruded film tube 81 has a circumference less than that established bythreshold C. Likewise, the maximum override signal MAO will remain highfor as long as the circumference of extruded film tube 81 remains largerthan the reference established by threshold B.

[0145] Threshold D and threshold E are also depicted in FIG. 15.Threshold D is established at a distance d 4 from reference R. ThresholdE is established at a distance d 5 from reference R. Thresholds D and Eare established by supervisory control unit 75, not by acoustictransducer 79. Threshold D represents a minimum circumference thresholdfor extruded film tube 81 which differs from that established bytransducer electronics 93. Likewise, threshold E corresponds to amaximum circumference threshold which differs from that established byacoustic transducer 79. Thresholds D and E are established in thesoftware of supervisory control unit 75, and provide a redundancy ofcontrol, and also minimize the possibility of user error, since thesethreshold are established in software, and cannot be easily changed oraccidentally changed. The coordination of all of these thresholds willbe discussed in greater detail below. In the preferred embodiment,threshold C is established at 10.8125 inches from reference R. ThresholdE is established at 3.6 inches from reference R.

[0146]FIG. 16 is a side view of the ultrasonic sensor 89 coupled tosizing cage 23 of the blown film tower 13, with permissible extrudedfilm tube 81 operating ranges indicated thereon. Setpoint A is thedesired distance between ultrasonic sensor 89 and extruded film tube 81.Thresholds D and C are established at selected distances inward fromultrasonic sensor 89, and 9 represent minimum circumference thresholdsfor extruded film tube 81. Thresholds B and E are established atselected distances from setpoint A, and establish separate maximumcircumference thresholds for extruded film tube 81. As shown in FIG. 16,extruded film tube 81 is not at setpoint A. Therefore, additional airmust be supplied to the interior of extruded film tube 81 to expand theextruded film tube 81 to the desired circumference established bysetpoint A.

[0147] If extruded film tube 81 were to collapse, two separate alarmconditions would be registered. One alarm condition will be establishedwhen extruded film tube 81 falls below threshold C. A second andseparate alarm condition will be established when extruded film tube 81falls below threshold D. Extruded film tube 81 may also becomeoverblown. In an overblown condition, two separate alarm conditions arepossible. When extruded film tube 81 expands beyond threshold B, analarm condition is registered. When extruded film tube 81 expandsfurther to extend beyond threshold E, a separate alarm condition isregistered.

[0148] As discussed above, thresholds C and B are subject to useradjustment through settings in transducer electronics 93. In contrast,thresholds D and E are set in computer code of supervisory control unit75, and are not easily adjusted. This redundancy in control guardsagainst accidental or intentional missetting of the threshold conditionsat transducer electronics 93. The system also guards against thepossibility of equipment failure in transducer 79, or gradual drift inthe threshold settings due to deterioration, or overheating of theelectronic components contained in transducer electronics 93.

[0149] Returning now to FIG. 14, operator control panel 137 andsupervisory control unit 75 will be described in greater detail.Operator control panel 137 includes setpoint display 109, which servesto display the distance d1 between reference R and setpoint A. Setpointdisplay 109 includes a 7 segment display. Distance selector 111 is usedto adjust setpoint A. Holding the switch to the “+” position increasesthe circumference of extruded film tube 81 by decreasing distance d1between setpoint A and reference R. Holding the switch to the “−”position decreases the diameter of extruded film tube 81 by increasingthe distance between reference R and setpoint A.

[0150] Target indicator 113 is a target light which displays informationpertaining to whether extruded film tube 81 is within range ofultrasonic transducer 89, whether an echo is received at ultrasonictransducer 89, and whether any error condition has occurred. Blowerswitch 139 is also provided in operator control panel 137 to allow theoperator to selectively disconnect the blower from the control unit. Asshown in FIG. 14, all these components of operator control panel 137 areelectrically coupled to supervisory control unit 75.

[0151] Supervisory control unit 75 responds to the information providedby acoustic transducer 79, and operator control panel 137 to actuateproportional valve 125. Proportional valve 125 in turn acts uponpneumatic cylinder 127 to rotate rotary valve 129 to control the airflow to the interior of extruded film tube 81.

[0152] With the exception of analog to digital converter 141, digital toanalog converter 143, and digital to analog converter 145 (which arehardware items), supervisory control unit 75 is a graphic representationof computer software resident in memory of supervisory control unit 75.In one embodiment, supervisory control unit 75 comprises an industrialcontroller, preferably a Texas Instrument brand industrial controllerModel No. PM550. Therefore, supervisory control unit 75 is essentially arelatively low-powered computer which is dedicated to a particular pieceof machinery for monitoring and controlling. In the preferredembodiment, supervisory control unit 75 serves to monitor many otheroperations of blown film extrusion line 11. The gauging and control ofthe circumference of extruded film tube 81 through computer software isone additional function which is “piggybacked” onto the industrialcontroller. Alternately, it is possible to provide an industrialcontroller or microcomputer which is dedicated to the monitoring andcontrol of the extruded film tube 81. Of course, dedicating amicroprocessor to this task is a rather expensive alternative.

[0153] For purposes of clarity and simplification of description, theoperation of the computer program in supervisory control unit 75 havebeen segregated into operational blocks, and presented as anamalgamation of digital hardware blocks. In the preferred embodiment,these software subcomponents include: software filter 149, emergencycondition control mode logic 150, health state logic 151, automaticsizing and recovery logic 153, loop mode control logic 155, volumesetpoint control logic 157, and output clamp 159. These software modulesinterface with one another, and to PI loop program 147 of supervisorycontrol unit 75. PI loop program is a software routine provided in theTexas Instruments' PM550 system. The proportional controller regulates aprocess by manipulating a control element through the feedback of acontrolled output. The equation for the output of a PI controller is: A

m=K*e+K/T∫e dt+ms

[0154] In this equation:

[0155] m=controller output

[0156] K=controller gain

[0157] e=error

[0158] T=reset time

[0159] dt=differential time

[0160] ms=constant

[0161] ∫e dt=integration of all previous errors

[0162] When an error exists, it is summed (integrated) with all theprevious errors, thereby increasing or decreasing the output of the PIcontroller (depending upon whether the error is positive or negative).Thus as the error term accumulates in the integral term, the outputchanges so as to eliminate the error.

[0163] CURRENT POSITION signal is provided by acoustic transducer 79 viaanalog output 99 to analog to digital converter 141, where the analogCURRENT POSITION signal is digitized. The digitized CURRENT POSITIONsignal is routed through software filter 149, and then to PI loopprogram 147. If the circumference of extruded film tube 81 needs to beadjusted, PI loop program 147 acts through output clamp 159 uponproportional valve 125 to adjust the quantity of air provided to theinterior of extruded film tube 81.

[0164]FIG. 17 is a schematic representation of the automatic sizing andrecovery logic ASRL of supervisory control unit 75. As stated above,this figure is a hardware representation of a software routine. ASRL 153is provided to accommodate the many momentary false indications ofmaximum and minimum circumference violations which may be registered dueto noise, such as the noise created due to air flow between acoustictransducer 79 and extruded film tube 81. The input from maximum alarmoverride MAO is “ored” with high alarm D, from the PI loop program, at“or” operator 191. High alarm D is the signal generated by the programin supervisory control unit 75 when the circumference of extruded filmtube 81 exceeds threshold D of FIG. 15. If a maximum override MAO signalexists, or if a high alarm condition D exists, the output of “or”operator 191 goes high, and actuates delay timer 193.

[0165] Likewise, minimum override MIO signal is “ored” at “or” operator195 with low alarm E. If a minimum override signal is present, or if alow alarm condition E exists, the output of “or” operator 195 goes high,and is directed to delay timer 197. Delay timers 193, 197 are providedto prevent an alarm condition unless the condition is held for 800milliseconds continuously. Every time the input of delay timers 193, 197goes low, the timer resets and starts from 0. This mechanism eliminatesmany false alarms.

[0166] If an alarm condition is held for 800 milliseconds continuously,an OVERBLOWN or UNDERBLOWN signal is generated, and directed to thehealth state logic 151. Detected overblown or underblown conditions are“ored” at “or” operator 199 to provide a REQUEST MANUAL MODE signalwhich is directed to loop mode control logic 155.

[0167]FIG. 18 is a schematic representation of the health-state logic151 of FIG. 14. The purpose of this logic is to control the targetindicator 113 of operator control panel 137. When in non-erroroperation, the target indicator 113 is on if the blower is on, and theTARGET PRESENT signal from digital output 105 is high. When an error issensed in the maximum override MAO or minimum override MIO lines, thetarget indicator 113 will flash on and off in one half second intervals.

[0168] In health-state logic HSL 151, the maximum override signal MAO isinverted at inverter 205. Likewise, the minimum override signal isinverted at inverter 207. “And” operator 209 serves to “and” theinverted maximum override signal MAO, with the OVERBLOWN signal, andhigh alarm signal D. A high output from “and” operator 209 indicatesthat something is wrong with the calibration of acoustic transducer 79.

[0169] Likewise, “and” operator 213 serves to “and” the inverted minimumoverride signal MIO, with the OVERBLOWN signal, and low alarm signal E.If the output of “and” operator 213 is high, something is wrong with thecalibration of acoustic transducer 79. The outputs from “and” operators209, 213 are combined in “or” operator 215 to indicate an error witheither the maximum or minimum override detection systems. The output of“or” operator 215 is channeled through oscillator 219, and inverted atinverter 217. “And” operator 211 serves to “and” the TARGET PRESENTsignal, blower signal, and inverted error signal from “or” operator 215.The output of “and” operator of 211 is connected to target indicator113.

[0170] If acoustic transducer 79 is properly calibrated, the target iswithin range and normal to the sonic pulses, and the blower is on,target indicator 113 will be on. If the target is within range andnormal to the sonic pulses, the blower is on, but acoustic transducer 79is out of calibration, target indicator 113 will be on, but will beblinking. The blinking signal indicates that acoustic transducer 79, andin particular transducer electronics 93, must be recalibrated.

[0171]FIG. 19 is a schematic representation of loop mode control logicLMCL of FIG. 14. The purpose of this software module is coordinate thetransition in modes of operation. Specifically, this software modulecoordinates automatic startup of the blown film extrusion process, aswell as changes in mode between an automated “cascade” mode and a manualmode, which is the required mode of the PI controller to enable underand overblown conditions of the extruded film tube 81 circumference. Theplurality of input signals are provided to loop mode control logic 155,including: BLOWER ON, REQUEST MANUAL MODE, PI LOOP IN CASCADE MODE,UNDERBLOWN and OVERBLOWN. Loop mode control logic LMCL 155 provides twooutput signals: MANUAL MODE, and CASCADE MODE.

[0172]FIG. 19 includes a plurality of digital logic blocks which arerepresentative of programming operations. “Or” operator 225 “ores” theinverted BLOWER ON SIGNAL to the REQUEST MANUAL MODE SIGNAL. “And”operator 227 “ands” the inverted REQUEST MANUAL MODE SIGNAL with aninverted MANUAL MODE SIGNAL, and the BLOWER ON SIGNAL. “And” operator229 “ands” the REQUEST MANUAL MODE SIGNAL to the inverted CASCADE MODESIGNAL. This prevents MANUAL MODE and CASCADE MODE from both being on atthe same time. “And” operator 231 “ands” the MANUAL MODE SIGNAL, theinverted UNDERBLOWN SIGNAL, and the OVERBLOWN SIGNAL. “And” operator 233“ands” the MANUAL MODE SIGNAL with the UNDERBLOWN SIGNAL. This causesthe overblown condition to prevail in the event a malfunction causesboth underblown and overblown conditions to be on. Inverters 235, 237,239, 241, and 243 are provided to invert the inputted output signals ofloop mode control logic 155 were needed. Software one-shot 245 isprovided for providing a momentary response to a condition. Softwareone-shot 245 includes “and” operator 247, off-delay 249, and inverter251.

[0173] The software of loop mode control logic 155 operates to ensurethat the system is never in MANUAL MODE, and CASCADE MODE at the sametime. When manual mode is requested by REQUEST MANUAL MODE, loop modecontrol logic 155 causes MANUAL MODE to go high. When manual mode is notrequested, loop mode control logic 155 operates to cause CASCADE MODE togo high. MANUAL MODE and CASCADE MODE will never be high at the sametime. Loop mode control logic 155 also serves to ensure that the systemprovides a “bumpless transfer” when mode changes occur. The term“cascade mode” is understood in the automation industries as referringto an automatic mode which will read an adjustable setpoint.

[0174] Loop mode control logic 155 will also allow for automatic startupof the blown film extrusion process. At startup, UNDERBLOWN SIGNAL ishigh, PI LOOP IN CASCADE MODE is low, BLOWER ON SIGNAL is high. Theseinputs (and inverted inputs) are combined at “and” operators 231, 233.At startup, “and” operator 233 actuates logic block 253 to move themaximum air flow value address to the Pi loop step 261. At startup, theMANUAL MODE SIGNAL is high. For the PI loop controller of the preferredembodiment, when MANUAL MODE is high, the value contained in PI loopoutput address is automatically applied to proportional valve 125. Thisresults in actuation of proportional valve 125 to allow maximum air flowto start the extruded film tube 81.

[0175] When extruded film tube 81 extends in size beyond the minimumthreshold (C and D of FIG. 15 ), the UNDERBLOWN SIGNAL goes low, and thePI LOOP IN CASCADE MODE signal goes high. This causes software one-shot245 to trigger, causing logic blocks 265, 267 to push an initial biasvalue contained in a program address onto the PI loop. Simultaneously,logic blocks 269, 271 operate to place the selected setpoint value Aonto volume-setpoint control logic VSCL 157. Thereafter, volume-setpointcontrol logic VSCL 157 alone serves to communicate changes in setpointvalue A to PI loop program 147.

[0176] If an overblown or underblown condition is detected for asufficiently long period of time, the controller will request a manualmode by causing REQUEST MANUAL MODE SIGNAL to go high. If REQUEST MANUALMODE goes high, loop mode control logic LMCL 155 supervises the transferthrough operation of the logic blocks.

[0177] Loop mode control logic LMCL 155 also serves to detectedoverblown and underblown conditions. If an overblown or underblowncondition is detected by the control system, REQUEST MANUAL MODE goeshigh, and the appropriate OVERBLOWN or UNDERBLOWN signal goes high. Thelogic operators of loop mode control logic LMCL 155 operate to overridethe normal operation of the control system, and cause maximum or minimumair flow by putting the maximum air flow address 261 or minimum air flowaddress 263 to the Pi output address. As stated above, when MANUAL MODEis high, these maximum or minimum air flow address values are outputteddirectly to proportional valve 125. Thus, when the extruded film tube 81is overblown, loop mode control logic LMCL 155 operates to immediatelycause proportional valve 125 to minimize air flow to extruded film tube81. Conversely, if an underblown condition is detected, loop modecontrol logic LMCL 155 causes proportional valve 125 to immediatelymaximize air flow to extruded film tube 81.

[0178]FIG. 20 depicts the operation of volume-setpoint control logicVSCL 157.

[0179] Volume setpoint control logic VSCL 157 operates to increase ordecrease setpoint A in response to changes made by the operator atdistance selector 111 of operator control panel 137, when the Pi loopprogram 147 is in cascade mode, i.e. when PI LOOP IN CASCADE MODE signalis high. The INCREASE SETPOINT, DECREASE SETPOINT, and PI LOOP INCASCADE MODE signals are logically combined at “and” operators 283, and287. These “and” operators act on logic blocks 285, 289 to increase ordecrease the setpoint contained in remote setpoint address 291. When thesetpoint is either increased or decreased, logic block 293 operates toadd the offset to the remote setpoint for display, and forwards theinformation to digital to analog converter 143, for display at setpointdisplay 109 of operator control panel 137. The revised remote setpointaddress is then read by the PI loop program 147.

[0180]FIG. 21 is a flowchart drawing of output clamp 159. The purpose ofthis software routine is to make sure that the PI loop program 147 doesnot over drive the rotary valve 129 past a usable limit. Rotary valve129 operates by moving a vane to selectively occlude stationaryopenings. If the moving vane is over driven, the rotary valve will beginto open when the PI loop calls for complete closure. In step 301, theoutput of the PI loop program 147 is read. In step 303, the output of Piloop is compared to a maximum output. If it exceeds the maximum output,the PI output is set to a predetermined maximum output in step 305. Ifthe output of PI loop does not exceed the maximum output, in step 307,the clamped PI output is written to the proportional valve 125 throughdigital to analog converter 145.

[0181] As shown in FIG. 14, emergency condition control mode logic 150is provided in supervisory control unit 75, and is shown in detail inFIG. 22. As shown in FIG. 22, emergency condition control mode logic 150receives three input signals: the OVER BLOWN signal; the UNDERBLOWNsignal; and the TARGET filter signal. The emergency condition controlmode logic 150 provides as an output two variables to software filter149, including: “SPEED HOLD”; and “ALIGN HOLD”. The OVERBLOWN signal isdirected to anticipation state “or” gate 403 and to inverter 405. TheUNDERBLOWN signal is directed to anticipation state “or” gate 403 and toinverter 407. The TARGET signal is directed through inverter 401 toanticipation state “or” gate 403, and to “and” gate 409. The output ofanticipation “or” gate 403 is the “or” combination of OVERBLOWN signal,and the inverted TARGET signal. Anticipation state “or” gate 403 and“and” gate 419 cooperate to provide a locking logic loop. The output of“or” gate 403 is provided as an input to “and” gate 419. The other inputto “and” gate 419 is the output of inverter 417. The output of inverter417 can be considered as a “unlocking” signal. If the OVERBLOWN signalor UNDERBLOWN signal is high, or the inverted TARGET signal is high, theoutput of anticipation state “or” gate 403 will go high, and will be fedas an input into “and” gate 419, as stated above. The output ofanticipation state “or” gate 403 is also provided as an input to “and”gates 413, 411, and 409. The other input to “and” gate 413 is theinverted OVERBLOWN signal. The other input to “and” gate 411 is theinverted UNDERBLOWN signal. The other input to “and” gate 409 is theTARGET signal. The outputs of “and” gates 409, 411, and 413 are providedto “or” gate 415. The output of “or” gate 415 is provided to inverter417.

[0182] In operation, the detection of an overblown or underblowncondition, or an indication that the extruded film tube is out of rangeof the sensor will cause the output of anticipation state “or” gate 403to go high. This high output will be fed back through “and” gate 419 asan input to anticipation state “or” gate 403. Of course, the output of“and” gate 419 will be high for so long as neither input to “and” gate419 is low. Of course, one input to “and” gate 419 is high because achange in the state of the OVER BLOWN signal, the UNDER BLOWN signal,and the TARGET signal has been detected. The other input to “and” gate419 is controlled by the output of inverter 417, which is controlled bythe output of next-state “or” gate 415. As stated above, the output ofnext-state “or” gate 415 is controlled by the output of “and” gates 409,411, 413. In this configuration, anticipation state “or” gate 403 and“and” gate 419 are locked in a logic loop until a change is detected ina binary state of one of the following signals: the OVERBLOWN signal,the UNDERBLOWN signal, and the TARGET signal. A change in state of oneof these signals causes next-state “or” gate 415 to go high, whichcauses the output of inverter 417 to go low, which causes the output of“and” gate 419 to go low.

[0183] The output of next-state “or” gate 415 is also provided to timerstarter 421, the reset pin for timer starter 421, and the input of block423. When a high signal is provided to the input of timer starter 421, athree second software clock is initiated. At the beginning of the threesecond period, the output of timer starter 421 goes from a normally highcondition to a temporary low condition; at the end of the three secondsoftware timer, the output of timer starter 421 returns to its normallyhigh condition. If any additional changes in the state of the OVERBLOWNsignal, the UNDERBLOWN signal, and the TARGET signal are detected, thesoftware timer is reset to zero, and begins running again. Theparticular change in the input signal of the OVERBLOWN signal, theUNDERBLOWN signal, and the TARGET signal, also causes the transmissionof a high output from “and” gates 409, 411, and 413 to blocks 429, 427,and 425 respectively.

[0184] In operation, when the input to block 423 goes high, the numericvalue associated with the variable identified as “quick filter align”will be pushed to a memory variable identified as “speed hold”. “Quickfilter align” is a filter variable which is used by software filter 149(of FIG. 23, which will be discussed below), which determines themaximum allowable rate of change in determining the estimated position.“Speed hold” is a holding variable which holds the numeric value for themaximum allowable rate of change in determining the estimated positionof the blown film tube. “Speed hold” can hold either a value identifiedas “quick filter align” or a value identified as “normal filter align”.“Normal filter align” is a variable that contains a numeric value whichdetermines the normal maximum amount of change allowed in determiningthe estimated position of the blown film tube relative to thetransducer. Blocks 423 and 431 are both coupled to block 433 which is anoperational block representative of a “push” operation. Essentially,block 433 represents the activity of continuously and asynchronouslypushing the value held in the variable “speed hold” to “LT2 ” insoftware filter 149 via data bus 402. The value for “normal filteralign” is the same as that discussed herebelow in connection with FIG.8a, and comprises thirteen counts, wherein counts are normalized unitsestablished in terms of voltage. The preferred value for “quick filteralign” is forty-eight counts. Therefore, when the software filter 149 isprovided with the quick filter align value, the control system is ableto change at a rate of approximately 3.7 times as fast as that during a“normal filter align” mode of operation.

[0185] Also, when a “locked” condition is obtained by anticipation state“or” gate 403 and “and” gate 419, any additional change in state of thevalues of any of the OVERBLOWN signal, the UNDERBLOWN signal, and theTARGET signal will cause “and” gates 409, 411, and 413 to selectivelyactivate blocks 429, 427, 425. Blocks 429, 427, and 425 are coupled toblock 433 which is linked by data bus 402 to software filter 149. Whenblock 429 receives a high input, the variable held in the memorylocation “target restore count” is moved to a memory location identifiedas “align hold”. When block 427 receives a high input signal, the valueheld in the memory location identified as “underblown count” is moved toa memory value identified as “align hold”. When block 425 receives ahigh input signal, the numeric value held in a memory locationidentified as “overblown count” is moved to a memory location identifiedas “align hold”. As stated above, block 433 performs a continuousasynchronous “push” operation, and will push any value identified to the“align hold” memory location to the values of SAMPLE (N), SAMPLE (N-1),and BPE in the software filter of FIG. 23. In the preferred embodimentof the present invention, the value of “overblown count” is set tocorrespond to the distance between reference R and maximum circumferencethreshold B which is depicted in FIG. 16, which is established distanceat which the control system will determine that an “overblown” conditionexists. Also, in the preferred embodiment of the present invention, thevalue of the “underblown” count will be set to a minimum circumferencethreshold C, which is depicted in FIG. 16, and which corresponds to thedetection of an underblown condition. Also, in the present invention,the value of “target restore count” is preferably established tocorrespond to the value of set point A, which is depicted in FIG. 16,and which corresponds generally to the distance between reference R andthe imaginary cylinder established by the position of the sizing cagewith respect to the blown film tube.

[0186]FIG. 23A is a flowchart of the preferred filtering process appliedto CURRENT POSITION signal generated by the acoustic transducer.Preferably, it includes multiple stages of filtering, for differentoperating conditions. The first stage of filtering pertains torelatively unstable operating conditions. The second stage of filteringpertains to relatively stable operating conditions. The digitizedCURRENT POSITION signal is provided from analog to digital converter 141to software filter 149. The program reads the CURRENT POSITION signal instep 161. Then, the software filter 149 sets SAMPLE (N) to the positionsignal.

[0187] In step 165, the absolute value of the difference between CURRENTPOSITION (SAMPLE (N)) and the previous sample (SAMPLE (N-1)) is comparedto a first threshold. If the absolute value of the difference betweenthe current sample and the previous sample is less than first thresholdT1, the value of SAMPLE (N) is set to CFS, the current filtered sample,in step 167. If the absolute value of the difference between the currentsample and the previous sample exceeds first threshold T1, in step 169,the CURRENT POSITION signal is disregarded, and the previous positionsignal SAMPLE (N-1) is substituted in its place.

[0188] Then, in step 171, the suggested change SC is calculated, bydetermining the difference between the current filtered sample CFS andthe best position estimate BPE. In step 173, the suggested change SCwhich was calculated in step 171 is compared to positive T2, which isthe maximum limit on the rate of change. If the suggested change iswithin the maximum limit allowed, in step 177, allowed change AC is setto the suggested change SC value. If, however, in step 173, thesuggested change exceeds the maximum limit allowed on the rate ofchange, in step 175, the allowed change is set to +LT2, a default valuefor allowed change.

[0189] In step 179, the suggested change SC is compared to the negativelimit for allowable rates of change, negative T2. If the suggestedchange SC is greater than the maximum limit on negative change, in step181, allowed change AC is set to negative −LT2, a default value fornegative change. However, if in step 179 it is determined that suggestedchange SC is within the maximum limit allowed on negative change, instep 183, the allowed change AC is added to the current best positionestimate BPE, in step 183. Finally, in step 185, the newly calculatedbest position estimate BPE is written to the PI loop program.

[0190] Software filter 149 is a two stage filter which first screens theCURRENT POSITION signal by comparing the amount of change, eitherpositive or negative, to threshold T1. If the CURRENT POSITION signal,as compared to the preceding position signal exceeds the threshold ofT1, the current position signal is discarded, and the previous positionsignal (SAMPLE (N-1)) is used instead. At the end of the first stage, instep 171, a suggested change SC value is derived by subtracting the bestposition estimate BPE from the current filtered sample CFS.

[0191] In the second stage of filtering, the suggested change SC valueis compared to positive and negative change thresholds (in steps 173 and179 ). If the positive or negative change thresholds are violated, theallowable change is set to a preselected value, either +LT2, or −LT2. Ofcourse, if the suggested change SC is within the limits set by positiveT2 and negative T2, then the allowable change AC is set to the suggestedchange SC.

[0192] As is shown in FIG. 23A, data bus 201 couples the emergencycondition control logic block 150 to software filter 149. As statedabove, emergency condition control logic block 150 is designed toasynchronously push a numeric value identified in the memory location of“speed hold” to LT2 in software filter 149. Furthermore, emergencycondition control logic block 150 will asynchronously push a numericvalue in the memory location identified as “ALIGN HOLD” to SAMPLE (N),SAMPLE (N-1), and BPE. As stated above, SAMPLE N corresponds to thecurrent position signal as detected by the transducer. SAMPLE (N-1)corresponds to the previous position signal as determined by thetransducer. BPE corresponds to the best position estimate.

[0193] Since the operation of emergency condition control mode logicblock 150 is asynchronous, block 186 of FIG. 23A should be read andunderstood as corresponding to an asynchronous read function. Therefore,at all times, as set forth in block 186, software filter 149 receivesvalues of “speed hold” and “align hold” from emergency condition controlmode logic block 150, and immediate substitutes them into the variouslogic blocks found in software filter 149. For example, SAMPLE (N) isfound in logic blocks 163, 165, and 167. SAMPLE (N-1) is found in logicblocks 165, and 169. BPE is found at logic block 183. The programfunction represented by block 186 operates to asynchronously andimmediately push the values of “speed hold” and “align hold” to thesevarious functional blocks, since OVERBLOWN, UNDERBLOWN, and lost TARGETconditions can occur at any time.

[0194] The normal operation of software filter 149 may also beunderstood with reference to FIG. 24, and will be contrasted withexamples of the emergency condition mode of operation as depicted inFIGS. 25, 26, and 27. In the graph of FIG. 24, the y-axis represents thesignal level, and the x-axis represents time. The signal as sensed byacoustic transducer 79 is designated as input, and shown in the solidline. The operation of the first stage of the software filter 149 isdepicted by the current filtered sample CFS, which is shown in the graphby cross-marks. As shown, the current filtered sample CFS operates toignore large positive or negative changes in the position signal, andwill only change when the position signal seems to have stabilized for ashort interval. Therefore, when changes occur in the current filteredsample CFS, they occur in a plateau-like manner.

[0195] In stage two of the software filter 149, the current filteredsample CFS is compared to the best position estimate BPE, to derive asuggested change SC value. The suggested SC is then compared to positiveand negative thresholds to calculate an allowable change AC which isthen added to the best position estimate BPE. FIG. 24 shows that thebest position estimate BPE signal only gradually changes in response toan upward drift in the POSITION SIGNAL. The software filtering system149 of the present invention renders the control apparatus relativelyunaffected by random noise, but capable of tracking the more “gradual”changes in bubble position.

[0196] Experimentation has revealed that the software filtering systemof the present invention operates best when the position of extrudedfilm tube 81 is sampled between 20 to 30 times per second. At thissampling rate, one is less likely to incorrectly identify noise as achange in circumference of extruded film tube 81. The preferred samplingrate accounts for the common noise signals encountered in blown filmextrusion liner.

[0197] Optional thresholds have also been derived throughexperimentation. In the first stage of filtering, threshold T1 isestablished as roughly one percent of the operating range of acoustictransducer 79, which in the preferred embodiment is twenty-one meters(24 inches less 3 inches). In the second stage of filter, thresholds+LT2 and −LT2 are established as roughly 0.30% of the operating range ofacoustic transducer 79.

[0198]FIG. 25A is a graphic depiction of the control system response tothe detection of an UNDERBLOWN condition. The X-axis of the graph ofFIG. 25A is representative of time in seconds, and the Y-axis of thegraph of FIG. 25A is representative of position in units of voltagecounts. A graph of the best position estimate BPE is identified bydashed line 503. A graph of the actual position of the extruded filmtube with respect to the reference position R is indicated by solid line501. On this graph, line 505 is indicative of the boundary establishedfor determining whether the blown film tube is in an “underblown”condition. Line 507 is provided as an indication of the normal positionof the blown film tube. Line 509 is provided to establish a boundary fordetermining when a blown film tube is considered to be in an “overblown”condition.

[0199] The activities represented in the graph of FIG. 25A may becoordinated with the graph of FIG. 25B, which has an X-axis which isrepresentative of time in seconds, and a Y-axis which represents thebinary condition of the TARGET signal, and the UNDERBLOWN signal, aswell as the output of block 421 of FIG. 22, which is representative ofthe output of the time out filter realignment software clock. Now, withsimultaneous reference to FIGS. 25A and 25 B, segment 511 of the bestposition estimate indicates that for some reason the best positionestimate generated by software filter 149 is lagging substantiallybehind the actual position of the blown film tube. As shown in FIG. 25A,both the actual and estimated position of the blown film tube are in anunderblown condition, which is represented in the graph of FIG. 25B.

[0200] As stated above, in connection with FIG. 22 and the discussion ofthe operation of the emergency condition control logic block 150, thelocking software loop which is established by anticipation state “or”gate 403 and “and” gate 419 will lock the output of anticipation state“or” gate 403 to a high condition. Therefore, next-state “or” gate 415is awaiting the change in condition of any of the following signals: theOVERBLOWN signal, the UNDERBLOWN signal, and the TARGET signal. As shownin FIG. 25A, at a time of 6.5 seconds, the actual position of the blownfilm tube comes within the boundary 505 established for the underblowncondition, causing the output of next-state “or” gate 415 to go high,which causes the output of inverter 417 to go low, which causes theoutput of “and” gate 419 to go low. This change in state also starts thesoftware timer of block 421, and causes block 427 to push the value of“underblown count” to the “align hold” variable. Also, simultaneously,software block 423 pushes the value of “quick filter align” to the“speed hold” variable. The values of “speed hold” and “underblown count”are automatically pushed to block 433. Meanwhile, the software timer ofblock 421 overrides the normal and continuous pushing of “normal filteralign” to the “speed hold” variable for a period three seconds. Thethree second period expires at 9.5 seconds.

[0201] Thus, for the three second time interval 513, software filter 149is allowed to respond more rapidly to change than during normaloperating conditions. As shown in FIG. 22, block 433 operates toautomatically and asynchronously push the value of “speed hold” to “LT2” in software filter 149. Simultaneously, block 433 operates tocontinuously, automatically, and asynchronously push the value of “alignhold” to SAMPLE (N), SAMPLE (N-1) and BPE in software filter 149. Thisoverriding of the normal operation of software filter 149 for a threesecond interval allows the software best position estimate 503 to catchup with the actual position 501 of the blown film tube. The jumprepresented by segment 515 in the best position estimate 503 of theblown film tube is representative of the setting of SAMPLE (N), SAMPLE(N-1) and BPE to the “underblown count” which is held in the “alignhold” variable. Segment 517 of the best position estimate 503 representsthe more rapid rate of change allowable during the three secondinterval, and depicts the best position estimate line 503 tracking theactual position line 501 for a brief interval. At the expiration of thethree second interval, software filter 149 of the control system returnsto a normal mode of operation which does not allow such rapid change inthe best position estimate.

[0202]FIGS. 26A and 26 b provide an alternative example of the operationof the emergency condition control mode of operation of the presentinvention. In this example, the TARGET signal represented in segment 525of FIG. 26b is erroneously indicating that the blown film tube is out ofrange of the transducer. Therefore, segment 529 of dashed line 527indicates that the best position estimate according to software filter149 is set at a default constant value indicative of the blown film tubebeing out of range of the transducer, and is thus far from indicative ofthe actual position which is indicated by line 531. This condition mayoccur when the blown film tube is highly unstable so that theinterrogating pulses from the transducer are deflected, preventingsensing of the blown film tube by the transducer. Segment 533 of FIG.26b is representative of stabilization of the blown film tube andtransition of the TARGET signal from an “off” state to an “on” state.This transition triggers initiation of the three second software timerwhich is depicted by segment 535. The time period begins at 12.5 secondsand ends at 15.5 seconds. The transition of the TARGET signal from a lowto a high condition triggers the pushing of the “target restore count”value to the “align hold” variable, as is graphically depicted bysegment 537. During the three second interval, the best positionestimate established by software filter 149 is allowed to change at arate which is established by the “quick filter align” value which ispushed to the “speed hold” variable and bused to software filter 149. Atthe termination of the three second interval, the software filter 149returns to normal operation.

[0203]FIG. 27A provides yet another example of the operation of theemergency condition control mode. Segment 541 of FIG. 27B indicates thatthe TARGET signal is in a low condition, indicating that the blown filmtube is out of range of the transducer. Segment 543 indicates that theblown film tube has come into range of the transducer, and the TARGETsignal goes from a low to a high condition. Simultaneous with themovement of the blown film tube into range of the transducer, theUNDERBLOWN signal goes from a low to a high condition indicating thatthe blown film tube is in an underblown condition. Segment 545 of FIG.27B indicates a transition from a high UNDERBLOWN signal to a lowUNDERBLOWN signal, which indicates that the blown film tube is no longerin an underblown condition. This transition initiates the three secondinterval which allows for more rapid adjustment of the best positionestimate.

[0204] The foregoing description related to the first stage of filteringwhich is especially useful during relatively unstable operatingconditions, wherein overblown and underblown extruded film tubeconditions are possible. The second stage of filtering, which will nowbe described, pertains to relatively stable operating conditions, whenthe extruded film tube is in a substantially fixed position. This typeof filtering is preferably a dynamic filtering operation, in which theinfluence of the dynamic filter is increased or decreased, dependingupon at least one pre-established criterion. Preferably, the criterioncomprises a comparison of the output of the filtering operation with thecurrent bubble position. If there is a great difference between thedetected extruded film tube position and the output of the filter, theoperating assumption is that the extruded film tube is perhaps becomingunstable, and the influence of the dynamic filtering operation should bereduced. Conversely, if the difference between the output of the dynamicfiltering process and the current position of the extruded film tube issmall or decreasing, the assumption is made that the extruded film tubeis in a relatively stable operating condition, and the influence of thedynamic filtering operation should be increased. In the presentinvention, the dynamic filtering operation comprises a rolling averageof detected position signals, with the number of samples utilized tocalculate the rolling average increasing if stability is detected anddecreasing if instability is detected. The foregoing will become clearwith reference to FIGS. 28A, 28 B, 28 C, 28 D, 28 E, 28 F, and 28 G.

[0205] With reference to FIG. 23A, the basic filtering operation isdepicted in flowchart form. At the termination of software step 183, abest position estimate (BPE) is calculated. The process continues atsoftware block 184(a) of FIG. 23B, wherein the best position estimate isprovided. Next, in accordance with software block 184(b), it isdetermined whether or not an alarm condition exists; if an alarmcondition exists, the process continues at software block 184(c),wherein the process continues by going to block 185 of FIG. 23A; if,however, it is determined in software block 184(b) that there is noalarm condition, the process continues. In software block 184(d), theprocessor determines whether or not the extruded film tube is in astartup mode of operation; if so, the process continues at softwareblock 184(e) by passing control to software block 185 in FIG. 23A;however, if it is determined in software block 184(d) that the bubble isnot a startup mode of operation, the process continues. In softwareblock 184(f), the controller determines whether or not there is anongoing change in extruded film tube balance; if so, the processcontinues at software block 184(g) by passing control to software block185 in FIG. 23A. However, if it is determined in software block 184(f)that there is no ongoing change in extruded film tube balance, theprocess continues. In accordance with software block 184(h), thecontroller determines whether the extruded film tube (or “bubble”) hasbeen stable for sixty continuous seconds; if not, the process continuesat software block 184(i), wherein control is passed to software block185 in FIG. 23A; however, if it is determined in software block 184(h)that the bubble has been stable for sixty continuous seconds, thencontrol is passed to software block 184(j), wherein the dynamic filterof FIG. 23C is utilized to process the position signals during thisrelatively stable interval of operation.

[0206] In broad overview, the basic filtering operation of FIG. 23Aalone is performed if any one of a variety of indicators reveal thatstable operation is not ongoing or is unlikely. A variety of therudimentary indicators are identified in FIG. 23B, and various otherindicators can be devised which can be added to the items in FIG. 23Bwhich provide further screening which prevents the dynamic filteringoperation from commencing.

[0207] Once relatively stable operations are ongoing, the dynamicfiltering operation may be applied. The preferred embodiment of thedynamic filtering operation is depicted in block diagram form in FIG.23C. As is shown, the process continues at software block 184(k),wherein the best position estimate is provided as an input to a rollingaverage generator 184(l) which computes a rolling average from a numberof previous samples of the best position estimate (BPE), preferablybased upon the following formula:

RA=RA+((BPE—RA_(prev))÷(Sample Number))

[0208] wherein RA is the rolling average; RA_(prev) is previous rollingaverage; BPE is the best position estimate currently provided; andSample Number is a number which determines the number of samplesutilized to calculate the rolling average

[0209] The output of rolling average generator 184(l) is subtracted fromthe input to the rolling average generator 184(l), which is the bestposition estimate (BPE). This defines an “ERROR”. This is provided as aninput to the number of samples calculator 184(m), which calculates thenumber of samples based upon the ERROR (which is input), a predeterminedGAIN value, and a BIAS value in accordance with the following formula:

SAMPLE NUMBER=(ERROR×GAIN)+BIAS

[0210] The BIAS 184(n) is a manufacturer-configurable variable whichhelps to determine the span (or range) of available sample numbersutilized in determining the rolling average. The output of the number ofsamples calculator 184(m) is provided as an input to software block184(o), which pushes the Sample Number to the rolling average generator184( 1 ) every second.

[0211] In accordance with present invention, the values for ERROR, GAINand BIAS are selected to insure that, during very stable operations, therolling average generator 184(l) utilizes ten (10) previous samples ofthe best position estimate (BPE) in order to calculate the rollingaverage. If the difference between the input to the rolling averagegenerator 184(l) and the output of the rolling average generator 184(l)increases, the number of samples calculator 184(m) reduces the number ofsamples utilized by the rolling average generator 184(i). When thedifference (ERROR) is at its greatest (and most unacceptable) level, thenumber of samples calculator 184(m) reduces the number of samples tounity (1), therefore causing the input of the rolling average generator184( 1 ) to be provided as the output of rolling average generator184(l) without any dynamic filtering whatsoever. In other words, as theERROR increases, the influence of the rolling average generator 184(l)is incrementally decreased from its maximum influence to its minimuminfluence, which essentially bypasses the dynamic filtering operationaltogether.

[0212] As is shown in FIG. 23C, the output of the rolling averagegenerator 184(l) is supplied to software block 184(p), which sets theBPE to the output of the rolling average generator 184(l). Then, inaccordance with 184(q), controls return to software block 185 of FIG.23A.

[0213] The beneficial influence of the dynamic filtering operation canbest be understood with reference to FIGS. 23D and 23 E. FIG. 23D is agraphically depiction of the bubble position 184(r) and the valveposition 184(s) with respect to time, without dynamic filtering. As isshown, the valve position moves in direct correspondence with the bubbleposition, quite dynamically. FIG. 23E is a graphical depiction of bubbleposition 184(t) and the output of the rolling average generator 184(u),as well as valve position 184(v), all with respect to time. As is shown,the rolling average generator is much more stable than the detectedbubble position (BPE). The extreme positive and negative peaks of thebubble position (BPE) are eliminated through the dynamic filteringprocess, making the control system altogether less susceptible to noiseand meaningless bubble flutter than without the dynamic filteringprocess. As is shown in FIG. 23(E), the valve (or other flow controldevice) is basically controlled by the output of the rolling averagegenerator, and is also much less susceptible to the noise or bubbleflutter. This type of noise is a common problem in particularly stiffmaterials, such as nylon.

[0214]FIG. 23F is a graphical depiction of a frequency distributioncomparison of the dynamically filtered position signal shown in singlecross-hatching and the unfiltered position signal (BPE) shown in doublecross-hatching. This frequency distribution reveals that there is abouta 33% reduction in the standard deviation between the dynamicallyfiltered position signal and the filtered, but not dynamically filtered,position signal. In the real world, this relates to about a 2 millimeterreduction in lay flat variation, which reduces a 6 millimeter totalvariation to about a 4 millimeter total variation. This greatlyincreases the control system's performance during these relativelystable operating intervals.

[0215]FIG. 23G is a graphical depiction of startup operations with thedynamic filter in place. The X-axis represents time and the Y-axisrepresents the valve position 184(w), the bubble position 184(x), theoutput of the rolling average generator 184(y). As is shown, the dynamicfiltering operation is not active until time 184(z), after which theprerequisite stability has been obtained. It is at that point that theposition of the valve 184(w) is directly controlled through the rollingaverage generator. Note the greater stability of valve position once therolling average generator has been activated.

[0216]FIG. 28 is a schematic and block diagram representation of anairflow circuit for use in a blown film extrusion system. Input blower613 is provided to provide a supply of air which is routed into airflowcircuit 611. The air is received by conduit 615 and directed to airflowcontrol device 617 of the present invention. Airflow control device 617operates as a substitute for a conventional rotary-type airflow valve631, which is depicted in simplified form also in FIG. 28. The preferredairflow control device 617 of the present invention is employed toincrease and decrease the flow of air to supply distributor box 619which provides an air supply to annular die 621 from which blown filmtube 623 extends upward. Air is removed from the interior of blown filmtube 623 by exhaust distributor box 625 which routes the air to conduit627, and eventually to exhaust blower 629.

[0217] The preferred airflow control device 617 is depicted infragmentary longitudinal section view in FIG. 29. As is shown, airflowcontrol device 617 includes housing 635 which defines inlet 637 andoutlet 639 and airflow pathway 641 through housing 635. A plurality ofselectively expandable flow restriction members 671 are provided withinhousing 635 in airflow pathway 641. In the view of FIG. 29,selectively-expandable flow restriction members 673, 675, 677, 679, and681 are depicted. Other selectively-expandable flow restriction membersare obscured in the view of FIG. 29. Manifold 685 is provided to routepressurized air to the interior of selectively-expandable flowrestriction members 671, and includes conduit 683 which couples to aplurality of hoses, such as hoses 687, 689, 691, 693, 695 which aredepicted in FIG. 29 (other hoses are obscured in FIG. 29).

[0218] Each of the plurality of selectively-expandable flow restrictionmembers includes an inner air-tight bladder constructed of an expandablematerial such as an elastomeric material. The expandable bladder issurrounded by an expandable and contractible metal assembly. Preferably,each of the plurality of selective-expandable flow restriction membersis substantially oval in cross-section view (such as the view of FIG.29), and traverse airflow pathway 641 across the entire width of airflowpathway 641. Air flows over and under each of the plurality ofselectively-expandable airflow restriction members, and each of themoperates as an choke to increase and decrease the flow of air throughhousing 635 as they are expanded and contracted. However, the flowrestriction is accomplished without creating turbulence in the airflow,since the selectively-actuable flow restriction members are foil shaped.

[0219] Returning now to FIG. 28, airflow control device 617 is coupledto proportional valve 657 which receives either a current or voltagecontrol signal and selectively vents pressurized fluid to airflowcontrol device 617. In the preferred embodiment, proportional valve 657is manufactured by Proportion Air of McCordsville, Ind. Supply 651provides a source of pressurized air which is routed through pressureregulator 653 which maintains the pressurized air at a constant 30pounds per square inch of pressure. The regulated air is directedthrough filter 655 to remove dust and other particulate matter, and thenthrough proportional valve 657 to airflow control device 617.

[0220] In the preferred embodiment of the present invention, airflowcontrol device 617 is manufactured by Tek-Air Systems, Inc. ofNorthvale, N.J., and is identified as a “Connor Model No. PRDPneumavalve”. This valve is the subject matter of at least two U.S.patents, including U.S. Pat. No. 3,011,518, which issued in December of1961 to Day et al., and U.S. Pat. No. 3,593,645, which issued on Jul.20, 1971, to Day et al., which was assigned to Connor EngineeringCorporation of Danbury, Conn., and which is entitled “Terminal Outletfor Air Distribution”, both of which are incorporated herein byreference as if fully set forth.

[0221] Experiments have revealed that this type of airflow controldevice provides for greater control than can be provided by rotary typevalve 631 (depicted in FIG. 28 for comparison purposes only), and isespecially good at providing control in mismatched load situations whichwould ordinarily be difficult to control economically with a rotary typevalve.

[0222] A number of airflow control devices like airflow control device617 can be easily coupled together in either series or parallelarrangement to control the total volume of air provided to a blown filmline or to allow economical load matching. In FIG. 28, a series and aparallel coupling of airflow control devices is depicted in phantom,with airflow control devices 681, 683, and 685 coupled together withairflow control device 617. As shown in the detail airflow controldevice 617 is in parallel with airflow control device 683 but is inseries communication with airflow control device 685. Airflow controldevice 685 is in parallel communication with airflow control device 681.Airflow control devices 681 and 683 are in series communication.

[0223] The present invention is also directed to a method and apparatusfor cooling extruded film tubes, which utilizes a mass air flow sensorto provide a measure of the flow of air in terms of both the air densityand air flow rate. The mass air flow sensor provides a numerical valuewhich is indicative of the mass air flow in an air flow path within ablown film extrusion system. A controller is provided for receiving themeasure of mass air flow from the mass air flow sensor and for providinga control signal to an adjustable air flow attribute modifier whichserves to selectively modify the mass air flow in terms of mass per unittime by typically changing one or more of the cooling air temperature,the cooling air humidity, or the cooling air velocity. The preferredmethod and apparatus for cooling extrude.film tubes is depicted anddescribed in detail in FIGS. 30 through 36, and the accompanying text.

[0224] The particular type of mass air flow sensor utilized in thepresent invention makes practical the utilization of mass air flowvalues in blown film extrusion systems. Of course, “mass air flow” issimply the total density of the cooling air or gas multiplied times theflow rate of the cooling air or gas. Typically, blown film extrusionlines utilize ambient air for cooling and/or sizing the molten blownfilm tube as it emerges from the annular die. It may become economicallypractical in the future to utilize gases other than ambient air; forpurposes of clarity and simplicity, in this detailed description and theclaims, the term “air” is intended to comprehend both ambient air aswell as specially provided gases or gas mixtures.

[0225] While it is simple to state what the “mass air flow” represents,it is far more difficult to calculate utilizing conventional techniques.This is true because of the difficulty associated with calculating thedensity of air. Air which contains water vapor requires the followinginformation for the accurate calculation of “mass air flow”: therelative humidity of the air, the absolute pressure of the air, thetemperature of the air, the saturation vapor pressure for the air at thegiven temperature, the partial pressure of the water vapor at the giventemperature, the specific gravity of the air, and the flow rate of theair. Utilizing conventional sensors, one could easily measure relativehumidity, temperature of the air, absolute pressure, and the flow rateof the air. With established data tables correlating the temperature ofthe gas and the relative humidity, the saturation vapor pressure and thepartial pressure of the water vapor can be calculated. For ambient airapplications, the specific gravity of the gas is unity so it drops outof consideration. A good overview of the complexity associated with thecalculation of these factors which make up the “mass air flow” isprovided in a book entitled Fan Engineering: An Engineers Handbook OnFans And Their Applications, edited by Robert Jorgensen, 8th edition,which is published by Buffalo Forge Company of Buffalo, N.Y. While suchcalculations are not particularly difficult given modern technologiesfor both sensors and data processors, the utilization of a single sensorwhich provides a direct indication of the “mass air flow” lessens thecosts associated with implementation of the method and apparatus forcooling extruded film tubes of the present invention. Such use of a massair flow sensor also reduces the complexity associated with calculatingmass air flow utilizing a more conventional technique. This can be seenby comparing the calculations required for a system which does notutilize a mass air flow sensor, with one which does utilize a mass airflow sensor. The “mass flow rate” of air is determined by equation 1.1which is set forth here below: Equation 1.1

Mass Flow Rate=Density*Flow Rate

[0226] Of course, the flow rate is easy to obtain from flow rate meters,but the density of the cooling air must be determined in accordance withequation 1.2 which is set forth here below: $\begin{matrix}{{Density} = \frac{\left( {\left( {P - {{Pws}\quad \phi}} \right) + {{Pws}\quad \phi \quad \omega}} \right)}{{.7543}\quad \left( {T + 459.7} \right)}} & {{Equation}\quad 1.2}\end{matrix}$

[0227] wherein P is representative of the absolute pressure of the air,Pws is representative of the saturation vapor pressure, ψ isrepresentative of the relative humidity, and ω is representative of theratio of the density of the water vapor to the density of dry air, and Tis representative of the temperature of the cooling air in degrees F.Since we measure P, ψ, and T directly, we only have to derive Pws and ω.By using a saturation vapor pressures table of water, we can determinethe saturation vapor pressure (Pws) from the temperature of the coolingair. The following equation 1.3 allows one to calculate ω, which is theratio of the water vapor density to dry air density: $\begin{matrix}{\omega = {1.6214 + \frac{\phi \quad ({Pws})^{1/1.42}}{1130}}} & {{Equation}\quad 1.3}\end{matrix}$

This formula is accurate to 0.1% in the range of temperatures from 32°F. to 400° F.

[0228] Therefore, it is evident that, in addition to a velocity sensor,sensors must be provided for the measurement of pressure, relativehumidity, and temperature. Additionally, the saturation vapor pressureand the ratio of the density of water vapor to the density of dry airmust be calculated utilizing a provided table, which in microprocessorimplementations must be represented by a data array maintained inmemory. All together, the complexity and opportunity for error presentedby such an array of sensors and series of calculations and table look-upoperations renders this technique difficult and expensive to implement.

[0229] In contrast, the present invention for cooling extruded tubesutilizes a single sensor which provides a direct measurement of the massair flow. Such mass air flow sensors have found their principleapplication in internal combustion engines, and are described andclaimed in the following issued United States patents, each of which isincorporated herein by reference as if fully set forth:

[0230] (1) U.S. Pat. No. 4,366,704, to Sato et al., entitled Air IntakeApparatus For Internal Combustion Engine, which issued on Jan. 4, 1983,and which is owned by Hitachi, LTD., of Tokyo, Japan;

[0231] (2) U.S. Pat. No. 4,517,837, to Oyama et al., entitled Air FlowRate Measuring Apparatus, which issued on May 21, 1985, and which isowned by Hitachi, LTD., of Tokyo, Japan;

[0232] (3) U.S. Pat. No. 5,048,327, to Atwood, entitled Mass Air FlowMeter, which issued on Sep. 17, 1991;

[0233] (4) U.S. Pat. No. 5,179,858, to Atwood, entitled Mass Air FlowMeter which issued on Jan. 19, 1993.

[0234] Mass air flow sensors operate generally as follows. One or more(typically platinum) resistor elements are provided in an air flow pathway. An energizing current is provided to the one or more resistorelements. Air passing over the resistor elements reduces the temperatureof the resistor elements. A control circuit is provided which maintainscurrents at a constant amount in accordance with King's Principal.

[0235] For the particular mass air flow sensor utilized in the preferredembodiment of the present invention, the mass air flow of the airflowing through an air pathway within a blown film extrusion system isestablished in accordance with equation 1.4 as follows:

[0236] Equation 1.4

Mass Flow Rate =al.601 (sensor reading+offset)^(c)

[0237] wherein the constants are attributable to the specificconstruction of the sensor assembly.

[0238] In accordance with the present invention, a mass air flow sensoris utilized to control air flow to cool molten polymers when extruded ina thin film tube. The air flow may be provided in contact with either aninterior surface of the thin film tube, an exterior surface of the thinfilm tube, or both an interior surface of the thin film tube and anexterior surface of the thin film tube. The air flow amount must beconsistent in order to maintain the desired cooling rate of the polymer.Changes in the cooling rate modify the extent to which polymer chainsare formed, linked, and cross-linked. Under the prior art, the coolingair is at best controlled to a constant temperature. There is noconsideration in prior art systems to the changes in the heat removingcapacity of the air as the air gets more or less humid, or as theabsolute pressure changes. Changes in the barometric pressure of oneinch of mercury can change the mass air flow rate by 3.3%. Changes inthe temperature in the air typically have the greatest effect on theheat removing capacity of the cooling air, with a 10% change in relativehumidity causing a tenth of 1% change in mass air flow rate. It isestimated that utilization of the present invention in blown filmextrusion lines which have temperature control will add an additionalaccuracy in cooling up to 3.5%. For blown film extrusion lines which donot have temperature control, the consistency in cooling can be improvedby an amount estimated at 13% to 15% provided physical limits of theattribute modifying equipment are not reached.

[0239] Cooling efficiency of course influences the production rate whichcan be obtained blown film extrusion lines. Generally speaking, it isdesirable to have the extruded molten material change in state from amolten state to a solid state before the blown film tube travels apredetermined distance from the annular die. In the industry, thelocation of the state change is identified as the “frost line’ in ablown film tube. In the prior art, when big changes occur in thetemperature, humidity, or barometric pressure, the frost line of theextruded film tube may move upward or downward relative to a desiredlocation. This may cause the operator of the blown film line to decreaseproduction volumes in order to keep from jeopardizing product quality,since product quality is in part determined by the position or locationof the frost line. While utilization of the present invention improvesthe cooling of extruded film tubes, the present invention also can beutilized to compensate for changes in the mass air flow rate of thecooling gas supplied to the interior of a blown film tube and the hotexhaust gas drawn from the blown film tube, to provide essentially aconstant frost line height, or at least a frost line height that doesnot move because of changes in the mass air flow rate. Of course, thepresent invention can be utilized in combination with prior art externalcooling devices for blown film extrusion lines to provide the samebenefit.

[0240] So considered broadly, the present invention can be utilized toaccomplish a number of desirable results, including:

[0241] (1) it can be used as a frost line leveler for blown filmextrusion line with external air cooling only;

[0242] (2) it can be used in both the supply and exhaust systems of aninternal-bubble-cooling blown film extrusion system to manage andmaintain a balanced air flow between the supply and exhaust, which couldgreatly stabilize the position of the frost line insofar as changes inthe ambient temperature, humidity, and barometric pressure effect theposition of the frost line; this could eliminate the need for prior artfrost line location sensors;

[0243] (3) the mass air flow sensor can be utilized in combination withthe controller or computer to determine the most effective and efficientoperating range of flow pump devices such as blowers, and fans, byallowing the computer to determine the mass air flow rate with relationto blower speed (and valve position) and then systematically eliminateundesirable ranges of operation, which are generally found at the lowestand highest ends of the operating range, where the flow pump or valvemay perform in a non-linear fashion which would introduce unstablecharacteristics into the operation of the blown film line;

[0244] (4) the mass air flow sensor can be utilized to provide a ratherslow feed back signal to a supply blower in the blown film line, tocompensate for changes in the ambient air, such as temperature,humidity, and barometric pressure, which effect the mass air flow rate;

[0245] (5) the mass air flow sensor can be used to provide a feed backloop which enhances the operation of a flow control valve in the line,to ensure that the valve operation is providing a particular air flowcharacteristic in response to a particular valve activation signal.

[0246] In the following detailed description, FIGS. 30 and 31 aredirected to a blown film extrusion system which includes an internalcooling air flow and an external cooling air flow. In contrast, thedetailed description relating to FIGS. 32 through 35 are directed to amore simple blown film extrusion system which includes only an externalcooling air flow.

[0247] With reference first to FIG. 30, there is depicted aninternal-bubble-cooling blown film extrusion line 701 in schematic form.As is shown, blown film tube 703 is extruded from annular die 705. Anultrasonic transducer 707 is utilized to gage the position of blown filmtube 703, and provides a control signal to position processor 709, allof which has been discussed in detail in this detailed description. Asizing cage 711 is provided to size and stabilize the blown film tube703. A flow of internal cooling air is supplied to the interior of blownfilm tube 703 through supply stack 713. As is conventional, exhauststack 717 is also provided in an interior position within blown filmtube 703 for removing the cooling air from the interior of blown filmtube 703. A cooling air is supplied to supply stack 713 through supplydistributer box 715, and the exhausted air is removed from blown filmtube 703 through exhaust distributor box 719. Additionally, an externalcooling air ring 721 is provided for directing a cooling stream of airto an exterior surface of blown film tube 703. Cooling air ring 721collaborates with the internal cooling air stream to change the state ofthe molten material from a molten state to a solid state. Cooling airring 721 is provided with entrained ambient air from air ring blower 723which may be set tot a flow rate either manually or automatically.

[0248] Supply distributor box 715 is provided with an entrained streamof cooling air in the following manner. Ambient air is entrained by theoperation of supply blower 729. It is received at input filter 725, andpassed through (optional) manual damper 727. If supply blower 729 is avariable-speed-drive type of supply blower, then manual damper 727 isnot required. Preferably, however, supply blower 729 is a variable speeddrive controller which provides a selected amount of air flow inresponse to a command received at a control input ofvariable-speed-drive 731. Also, preferably, variable speed drivecontroller is optionally subject to synchronous command signals from IBCcontroller 753 which controls the general operations of the blown filmextrusion line. The entrained ambient air is routed through air flowpath 755, first through cooling system 733, which preferably includes aplurality of heat exchange coils and heat transference medium incommunication with the air flow, which receives a circulating heatexchange medium (such as chilled water for transferring heat), past massair flow sensor 737, through air flow control device 739 (such as thatdepicted and described in connection with FIGS. 28 and 29 above), andthrough supply distributer box 715. Mass air flow sensor 737 provides avoltage signal which is indicative of the mass air flow of the airflowing through air flow path 755 in the region between cooling system733 and air flow control device 739. Air flow control device 739operates in response to proportional valve 741 and selectively receivescompressed air from compressed air supply 743. Air flow control device739 includes a plurality of members which may be expanded and contractedto enlarge or reduce the air flow path way through he housing of airflow control device. This allows for the matching of loads, as isdiscussed above in connection with FIGS. 28 and 29. Proportional valve741 is under the control of IBC controller 753.

[0249] Exhaust distributer box 719 removes cooling air from blown filmtube 703 and routes it through damper 745, into air flow path 755. Theair passes through mass air flow sensor 747 which provides a voltagewhich is indicative of the mass air flow of the exhaust from blown filmtube 703. The air is pulled from air flow path 755 by the operation ofexhaust blower 749 which is responsive to an operator command,preferably through a variable speed drive 751, which is also preferablyunder the synchronous control command of IBC controller 753.

[0250] In broad overview, mass air flow sensor 737 provides anindication of the mass air flow of the cooling air which is suppliedthrough supply distributor box 715 to supply stack 713. This cooling airremoves heat from blown film tube 703, helping it change from a moltenstate to a solid state. Mass air flow sensor 747 is in communicationwith the exhaust air removed through exhaust stack 717 and exhaustdistributor box 719. Mass air flow sensor 747 provides a voltage whichis indicative of the mass air flow of the exhaust cooling air. Themeasurements provided by mass air flow sensors 737,747 are supplied to acontroller which includes a microprocessor component for executingpreprogrammed instructions.

[0251] In accordance with the present invention, IBC controller 753compares the values from mass air flow sensors, 737, 747 and thenprovides command controls to variable speed drives 731, 751 in order toeffect the operation of supply blower 729 and/or exhaust blower 749.Preferably, IBC controller 753 may be utilized in response to anoperator command to maintain supply blower 729 and/or exhaust blower 749at a particular level or magnitude of blower operation, or to provide aparticular ratio of blower operation, so that when the temperature,humidity, or barometric pressure of the ambient air changessignificantly, the blowers adjust the flow rate of the input cooling airand exhaust cooling air to blown film tube 703 to maintain uniformity ofheat absorbing capacity of the internal cooling air, notwithstanding thechange in temperature, humidity, and/or barometric pressure.

[0252] The operation of this rather simple feed back loop is set forthin flowchart form in FIG. 36. The process starts at software block 771,and continues at software block 773, wherein IBC controller 753 receivesan operator command from either an operator interface 757 on IBCcontroller 753, or an operator interface 759 on variable speed drive731. Next, values provided by mass air flow sensors 737 and 747 arerecorded in memory, in accordance with software block 775. Then inaccordance with step 777, operation set points are derived. For example,a particular ratio between the mass air flow detected at mass air flowsensor 737 and mass air flow sensor 747 may be derived. Then, inaccordance with step 779, IBC controller 75 monitors signals from massairflow sensors 737 and 747 for changes in mass air flow, which areprincipally due to changes in the ambient temperature, humidity, andbarometric pressure. Once a change is detected, in accordance with step781 IBC controller 753 synchronously adjusts the variable speed drives759, 731, 751 in order to affect the value of the mass air flow ofambient air which has been entrained and which is flowing through airflow passage way 755 in a manner which returns operation to the setpoint values derived in step 777. For example, variable speed drive 731,751 may be utilized to increase or decrease the volume of air entrainedby supply blower 729 and/or exhausted by exhaust blower 749. Inaccordance with step 783, this process is repeated until an additionaloperator command is received. Such commands may include an instructionto obtain a new operation set point, or to discontinue the feed backloop until instructed otherwise. A cooling coil 738 may also be providedin communication with air flow path 745, and may be adjusted in responseto IBC controller 753 to adjust the value of mass air flow.

[0253]FIG. 31 depicts an alternative to the embodiment of FIG. 30wherein mass air flow sensors are utilized to control both the internalcooling air supply to the interior of blown film tube 703 and anexternal cooling air stream which is supplied to the exterior surface ofblown film tube 703 from air ring 721. The figures differ in that, inaddition of having a control system for internal cooling air, a controlsystem for external cooling air is also provided with a mass air flowsensor 747 positioned in air flow path 741 between air ring blower 723and cooling air ring 721. Mass air flow sensor 747 provides ameasurement of the mass air flow of the air flowing within air flow path745. This measurement is provided to IBC controller 753 and compared toa set point value which has been either manually entered by the operatorat operator interface 757 or which has been automatically obtained inresponse to an operator command made at operator interface 757. IBCcontroller 753 supplies a control signal to variable speed drive 744which is utilized to adjust the operating condition of air ring blowereither upward or downward in order to maintain the established setpoint. If the mass air flow sensor 747 indicates to IBC controller 753that the total mass air flow has been diminished (perhaps due to changesin temperature, humidity, and barometric pressure), then IBC controller753 may supply a command signal to variable speed drive 744 whichincreases the throughput of air ring blower 723 in a manner whichcompensates for the diminishment in mass air flow as detected by massair flow sensor 747. If mass air flow sensor 747 detects an increase inthe mass air flow, IBC controller 753 may provide a command signal tovariable speed drive 744 which increases the throughput of air ringblower 723 in a manner which compensates for the diminishment in massair flow a detected by mass air flow sensor 747. If mass air flow sensor747 detects an increase in the mass air flow, IBC controller 753 mayprovide a command signal to variable speed drive 744 which reduces thethroughput of air ring blower 723, thus diminishing the amount of massair flow in order to make it equal to the set point maintained in memoryin response to an operator command. This simple feedback loop is alsocharacterized by the flowchart depiction in FIG. 36. Since changes inambient temperature, ambient humidity, and barometric pressure arerather slow, it is not necessary that this feedback loop be a very fastloop. It is sufficient that every few minutes the value for the mass airflow sensor be monitored to determine the numeric value of the mass airflow, that this value be compared to a set point recorded in memory, andthat an appropriate command be provided to blower in order to adjust themass air flow upward or downward to make it equivalent to the set pointvalue. This allows a program which implement the present invention to be“piggy backed” onto the IBC controller 753. The calculations required tocompare mass air flow values to set points is trivial and theseoperations need only be performed every few minutes, so the IBCcontroller can spend the vast majority of its computational power ofcontrolling the blown film line, with only a de minimis portion expendedto occasional checking and adjusting of the mass air flow. Additionally,a cooling coil 74 may be provided in communication with air flow path745, and may be provided in communication with air flow path 745, andmay be adjusted in response to IBC controller 753 to adjust the value ofmass air flow.

[0254] The present invention can also be utilized in far simpler blowfilm extrusion systems which utilize only external cooling air to removeheat from a molten blown film tube. Four particular embodiments aredepicted in FIGS. 32, 33, 34, and 35. In each of these embodiments, amass air flow sensor is positioned intermediate and external cooling airring and a blower for entraining and supplying air to the cooling ring.Additionally an adjustable air flow attribute modifier is provided inthe air flow path for selectively modifying the air mass per unit time.This adjustable air flow attribute modifier may comprise any mechanismfor adjusting for modifying the mass air flow, but in particular willmost probably comprise a cooling coil system which chills the coolingair, or an air flow control device which restricts or enlarges thequantity of air available for entrainment by the supply blower, or afluid injection system which modifies the humidity of the cooling air.Each of these three principle alternative embodiments will be discussedin detail herebelow in connection with FIGS. 32, 33, 34, and 35.

[0255] Turning first to FIG. 32, an external cooling blown filmextrusion line is depicted in schematic form. Plastic pellets are loadedinto resin hopper 791, passed through heating apparatus 793, and drivenby extruder 795 through die 797 to form a molten extruded film tube 789,with a portion of the extruded film tube 789 below frost line 801 beingin a molten state, and that portion above frost line 801 being in asolid state. Air ring 799 is positioned adjacent die 797 and adapted toroute cooling air along the exterior surface of blown film tube 789. Airring 799 is supplied with cooling air which is entrained by air ringblower 803, routed through cooling coils 805 of cooling system 809, andthrough mass air flow sensor 807. Preferably, mass air flow sensor 807is positioned in air flow path 821 intermediate cooling coils 805 andexternal cooling air ring 799. Cooling coils 805 are adapted to receivechilled water 813 from chiller system 81 1. Controller 815 is providedfor receiving a signal from mass air flow sensor 807 which is indicativeof the mass air flow of the cooling air flowing through air flow path821, and for providing a command signal to chiller system 811 whichadjusts the temperature of chilled water 813 which is routed throughcooling coil 805. A feed back loop is established about a set pointselected by the operator when a set point selection command button 817is depressed. Controller 815 will respond to the command by recording inmemory the mass air flow value provided by mass air flow sensor 807, andby adjusting the chiller system 811 upward or downward in temperature inorder to maintain the mass air flow value of cooling air flowing throughair flow path 821 at a value established by the set point. Of course,the operator has an operator interface for chiller system 811 whichallows for the operator setting of the temperature of chiller system811. This system works once the operator has established that sufficientcooling has been obtained, and should provide an equivalent level ofcooling from the external cooling air provided by air ring 799 eventhough the ambient air changes its density through relatively slowchanges in temperature, humidity, and barometric pressure. Theembodiment of FIG. 32 is especially suited for blown film extrusionlines which have a dedicated chiller system. The embodiment of FIG. 33depicts a more common scenario, wherein a single chiller system isshared by multiple blown film lines. In this event, the configurationdiffers insofar as chiller system 811 is utilized to provide chilledwater 813 for delivery to multiple heat exchange cooling coils, with aflow valve, such as flow valve 825, being provided of each set of heatexchange cooling coils to increase or decrease the flow o circulatingheat exchange fluid in order to alter the temperature of the cooling airin air flow path 821. In the embodiment depicted in FIG. 33, controller815 provides an electrical command signal to an electrically-actuatedflow valve 825 in order to increase or decrease the flow of chilledwater 813 from chiller system 811 to cooling coil 805. Similar to theembodiment of FIG. 32, the operator instructs controller 815 to recordthe mass air flow value from mass air flow sensor 807, and to utilizethat as a set point for operation. Thereafter, changes in the mass airflow property of the cooling air passing through air flow path 821, suchas changes caused by changes in temperature, humidity, and barometricpressure, are accommodated by increasing or diminishing the flow ofchilled water from chiller system 811 to heat exchange cooling coil 805.Increases in mass air flow will result in the controller 815 providing acommand to electrically-actuated flow valve 825 to diminish the flow ofchilled water; in contrast, decreases in mass air flow as detected bymass air flow sensor 807 will result in controller 815 providing acommand signal to electrically-actuated flow valve 825 to increase theflow of chilled water from chiller system 811 to heat exchange coolingcoils 805.

[0256]FIG. 34 is a schematic depiction of an external air blown filmextrusion line, with blown film tube 789 extending upward from die 797and being cooled by an air stream in contact with an exterior surface ofblown film tube 789 which is provided by air flow path 821. Air flowpath 821 includes mass air flow sensor 807 which provides a numericalindication of the mass air flow of the air passing through air flow path821. It provides this numerical indication to controller 815, which inturn supplies a command signal to either variable speed controller 831or air flow control device 833 (such as that depicted in FIGS. 28 & 29above), each of which can effect the volume of air which is entrained byair ring blower 803. Controller 815 includes a manual control 817 whichis utilized by the operator to establish a set point of operation.Typically, the operator will get the blown film line operating in anacceptable condition, and then will actuate the set point command 817,causing controller 815 to record in memory the value provided by massair flow sensor 807. Thereafter, changes in the mass air flow due tochanges in temperature, humidity, or barometric pressure will becompensated for by variation in the amount of air entrained by air ringblower 803, in order to maintain mass air flow value at or about the setpoint value. For example, if the mass air flow value decreases, asdetermined by the mass air flow sensor 807, variable speed controller831 or air flow control device 833 are provided with command signalsfrom controller 815 to increase the volume of air flowing through airflow path 821; however, if the mass air flow value increases, asdetermined by mass air flow sensor 807, controller 815 provides acommand signal to either variable speed controller 831 or air flowcontrol device 833 in order to decrease the volume of air entrained byair ring blower 803. In this manner, controller 815 may intermittentlycheck the value of the mass air flow, compare it to a set point valuerecorded in memory, and adjust the volume of air entrained by air ringblower 803 in order to maintain a mass air flow value at or about theset point. In this manner, the cooling ability the air stream in contactwith the exterior of extruded film tube 789 is maintained at a constantlevel notwithstanding gradual or dramatic changes in temperature,humidity, and barometric pressure.

[0257]FIG. 35 depicts yet another embodiment of the invention, whereinan external cooling blown film extrusion line is depicted in theschematic form, with extruded film tube 789 extending upward fromannular die 797, which is cooled by an air stream provided by coolingair ring 799 Cooling air ring 799 receives its cooling air from air flowpath 821. Mass air flow sensor 807 is positioned in air flow path 821,and is adapted to provide a signal indicative of the mass air flow ofair flowing through this passage way, to controller 815. Controller 815provides a command signal to water injector 835 which is also incommunication with the air passing through air flow path 821. Waterinjector 835 is adapted to increase the humidity of the air entrained byblower 803 in response to a command from controller 815. In accordancewith this embodiment of this invention, the operator depresses a setpoint control 817 on controller 815 in order to establish a set point ooperation for controller 815. Controller 815 records in memory the valueof mass air flow sensor 807, and thereafter continuously monitors thevalues provided by mass air flow sensor 807 in comparison to the setpoint. When an increase in mass air flow is required, controller 815provides a command signal to water injector 835 which provides apredetermined amount of moisture which is immediately absorbed by theair entrained by air ring blower 803. When no additional humidity isrequired, controller 815 will no provide such a command. In this manner,the mass air flow value for air entrained in air flow path 821 may bemoderated by operation of controller 815. Since this system easilyallows an increase in the mass air flow value, without allowing acorresponding decrease in the mass air flow value, it is particularlyuseful in very hot and dry climates.

[0258] In all embodiments, it is advisable to provide a predeterminedtime interval of time interval of monitoring before the set point isrecorded and established. This allows the operator to make changes inthe operating condition of the various blowers and other equipment inthe blown film line prior to requesting that a set point be established.It takes many minutes (5, 10, or 20 minutes) in order for the system toreach a quiescent condition of operation. Having a predefined intervalof time after request for a set point, during which the mass air flowvalues are monitored but not recorded, allows the operator to change theoperating state of the blown film line, and request a set point value,at the same time, without obtaining a set point value which is perhapsnot stable or quiescent. In yet another more particular embodiment ofthe present invention, the controller may be programmed to monitor therate of change of the mass air flow value for predetermined timeinterval in order to determine for itself that a quiescent condition hasbeen obtained. For example, a 10 or 20 minute interval may be providedafter operator request of a set point, during which the controllercontinuously polls the mass air flow sensor, calculates a rate of changefor a finite time interval, and records it in memory. Only when the rateof change reaches an acceptable level will the controller determine thata quiescent interval has been obtained, and thereafter record the massair flow value in memory for utilization as a set point, or in thederivation of a set point, about which the feedback loop is established.

[0259]FIG. 37A is a pictorial and schematic representation of the priorart technique for controlling an extruded film tube during startupoperations. In the prior art, a linear ratio controller is utilized by ahuman operator in order to determine and set the balance condition of asupply blower and an exhaust blower in a blown film extrusion apparatus.The prior art operates by utilizing human-set potentiometers in order tobalance the supply component 1003, load component 1005, and the exhaustcomponent 1007 in a linear ratio controller 1001. Determining thebalance condition of a supply blower and an exhaust in a blown filmextrusion apparatus is complicated by the fact that the blowers arenon-linear. This is graphically depicted in FIGS. 37B and 37 C. FIG. 37Bis a graph 1009 of the response curve 1015 of a supply blower, with theX-axis 1011 representative of the air flow in units of cubic feet perminute, and the Y-axis 1013 representative of pressure in inches ofwater. As is shown in FIG. 37B, the response curve 1015 is not linear.FIG. 37C is a graph 1017 of the response curve 1023 of an exhaustblower, with the X-axis 1019 representative of air flow in cubic feetper minute, and the Y-axis 1021 representative of pressure in inches ofwater. As is clear from FIG. 37C, the response curve 1023 of the exhaustblower is not linear.

[0260]FIG. 37D is a schematic and block diagram representation of thestartup control apparatus 1030 of the present invention. As is shown, adie 1033 receives molten material and extrudes film tube 1031. Air issupplied to the interior of extruded film tube 1031 via supply inlet1035, and air is exhausted from extruded film tube 1031 through exhaustoutlet 137. During production operations, a balance between the supplyand exhaust must be maintained (in fact, the balance is slightly biasedtoward supply) in order to maintain the extruded film tube 1031 at apredetermined and substantially constant circumference. As is shown inFIG. 37D, supply blower 1037 communicates through air flow pathway 1036to supply air to the interior of extruded film tube 1031. In accordancewith the preferred embodiment of the present invention, valve member1034 is provided within air flow pathway 1036 in order to provide foradjustment of the supply in order to allow for fine control over thecircumference of the extruded film tube 1031, as has been discussed indetail above. Valve 1034 may comprise a rotary valve (as discussedabove) or an air flow control member which includesselectively-expandable flow restriction members (also as discussedabove). Exhaust blower 1039 communicates with the interior of extrudedfilm tube 1031 through air flow pathway 1038. Supply blower 1037 andexhaust blower 1039 are under the control of variable speed drive 1041and variable speed drive 1043 through control lines 1045, 1047. Supplycontrol signals are directed to supply blower 1037 via control line 1045to increase or decrease its output. Likewise, exhaust control signalsare directed via exhaust control line 1047 to exhaust blower 1039 inorder to increase or decrease its output. In accordance with the presentinvention, variable speed drives 1041, 1043 are under control ofcontroller 1077.

[0261] Controller 1077 communicates with variable speed drive 1041through stop/start line 1051 which stops/starts variable speed drive1041, flow matching signal 1053 which communicates a control signal tovariable speed drive 1041, actual speed line 1055 which provides anindication of the actual speed of supply blower 1037 to controller 1077,and OK switch 1049 which communicates through lines 1057, 1059 tocontroller 1077 which provides a signal to controller 1077 when variablespeed drive 1041 is operating correctly.

[0262] Controller 1077 communicates with variable speed drive 1043through stop/start line 1063 which provides a stop or start signal tovariable speed drive 1043, actual speed line 1065 which provides anindication of the actual speed of exhaust blower 1039 to controller1077, and master IBC signal 1067 which communicates through master speedreference 1071 (which is an operator-adjustable potentiometer) whichprovides operator input to controller 1077 regarding the operatingconditions of the supply blower 1077 and exhaust blower 1039 duringstartup operations. Additionally, controller 1037 is provided with astatus indication via lines 1073, 1079 and switch 1061 which provides anindication of the operating condition of variable speed drive 1043.

[0263] Controller 1077 communicates with control panel 1079 whichprovides data to the operator, and which allows for operator input andcommands. Control panel 1079 includes inlet on/off switch 1083 andoutlet on/out switch 1085 which allow the operator to stop and start thesupply blower 1037 and exhaust blower 1039. Preferably, control panel1079 also includes manual blower input means 1091 which allows formanual control of the blowers. Additionally, control panel 1079 includesa blower balance display 1087 and master speed display 1089. Preferably,and additionally, control panel 1079 includes decrease button 1090, andincrease button 1092, which allow the operator to manually adjust eitheror both the supplier blower 1037 and exhaust blower 1039 during certainoperations (but in the preferred embodiment, just the supply blower), aswill be discussed in detail below.

[0264] The startup control apparatus 1030 of the present inventionallows the operator to efficiently stabilize the extruded film tube byautomatically coordinating the flow rate of the supply blower 1045 withthe flow rate of the exhaust blower 1047. The startup control apparatus1030 provides a special startup feature that minimizes the need toestablish a separate setup of startup settings. Compensation fornon-linear blower curves is managed by a combination of learned settingsand an efficient means to verify the learned settings are stillaccurate. The startup control apparatus 1030 also includes a bubblebreak detector that allows the option of stopping the production linewhen a bubble break occurs. In accordance with the present invention,the startup control apparatus 1030 monitors the status of each of thesupply blower 1045 and exhaust blower 1047 and uses such status tomanage startup and shutdown.

[0265]FIG. 37E is a flowchart representation of some of the routinesutilized during startup procedures. The process begins at software block1100 and continues at software block 1101, wherein controller 1077determines whether a startup mode of operation has been selected; if so,control passes to software block 1103, wherein the startup procedure ofFIG. 37F(1) through 37 F(2) is performed. If it is determined insoftware 1101 that the startup mode has not been selected, controlpasses to software block 1105, wherein controller 1077 determineswhether the run mode has been selected; if the run mode has beenselected, control passes to software block 1107, where controller 1077performs the run procedures of FIGS. 37G through 37 J. If it isdetermined in software block 1105 that the run mode has not beenselected, control passes to software block 1109, which determineswhether the balance mode of operation has been selected. If the balancemode of operation has been selected, control passes to software block1111, wherein controller 1077 performs the balance procedure of FIG.37K. If it is determined in software block 1109 that the balance modehas not been selected, control passes to software block 1113, where theprocess ends.

[0266] Turning now to FIGS. 37F(1) and 37 F(2), the startup mode will beexplained with reference to the flowchart. The process begins atsoftware block 1121 and continues at software block 1123, wherein theoperator activates the inlet blower. Next, in accordance with softwareblock 1125, controller 1077 fetches a start percent parameter which isrecorded in memory. In accordance with the preferred embodiment of thepresent invention, the start percent parameter is a predeterminedpercentage of the value of the master speed control displayed on masterspeed display 1089 (of FIG. 37D). In accordance with software block1127, controller 1077 sends control signals through variable speed drive1041 to supply blower 1037 in order to ramp supply blower 1037 up to thestart percent parameter. In accordance with the present invention, apredetermined ramping function 1128 is stored in memory of controller1077 which provides a bumpless ramp function which is followed in theramping up of supply blower 1037. In the preferred embodiment of thepresent invention, the ramp function is non-linear to improve the blowerresponse and to reduce the chance of overshooting the start percentvalue. An example of the ramp function 1128 is depicted adjacentsoftware block 1127.

[0267] Next, in accordance with software block 1129, controller 1077deactivates an outlet blower stop circuit in order to allow exhaustblower 1039 to start up. Next, in accordance with software blocks 1131and 1133, controller 1077 determines whether the operator has adjustedthe master speed reference potentiometer 1071 (of FIG. 37D). If so, theinlet blower is adjusted in accordance with software block 1133. Theprocess continues at software block 1135, wherein the operatordetermines that the extruded film tube (or “bubble”) is through theroller nips (as is depicted in FIG. 1). Next, in accordance withsoftware block 1137, the operator activates the exhaust blower 1039 byactuating outlet on/off switch 1085. Next, in accordance with softwareblock 1139, controller 1077 ramps the exhaust blower 1039 (through apredetermined ramping function 1140, which is preferably linear) to thefull-rated value of the master speed reference potentiometer 1071 (ofFIG. 37D). Controller 1077 then monitors the speeds of the supply blower1037 and the exhaust blower 1039 in order to determine if the speeds aresubstantially equal, as set forth in software block 1141. If the speedsare not equal, monitoring and comparing operations continue. If it isdetermined in software block 1141 that the speeds of the supply blowerand the exhaust blower are equal, control passes to software block 1143,wherein the inlet blower is ramped (again, in accordance with apredetermined function 1144 which is preferably non-linear) to thefull-rated value of the master speed reference potentiometer 1071 (ofFIG. 37D) as displayed on master speed display 1089 (also of FIG. 37D).Next, in accordance with software block 1145, controller 1077 monitorsthe position of the extruded film tube. Next, and in accordance withsoftware block 1147, controller 1077 determines whether the extrudedfilm tube is within range of a predetermined sensor (preferably, thecage sensor). If the extruded film tube is not within range, controlpasses back to software block 1145; however, if the extruded film tubeis within a predetermined range, control passes to software block 1149,wherein controller 1077 is utilized to adjust the supply blower 1037 toplace the valve 1036 in the middle of its linear operating range.

[0268] In accordance with the present invention, valve 1036 may compriseeither a rotary valve or the “bladder” valve discussed above. Each ofthese valves has a preferred and substantially linear operating range,but the valves are generally not linear over their entire operatingrange. Therefore, in accordance with the present invention, the linearoperating range of a particular valve might be determined empirically ina laboratory, and controller 1077 will be programmed to maintain thevalve in its relatively linear operating range. When a “bladder” valveis utilized, that linear range represents a closure condition in therange of 28% to 32%. Operation outside of that narrow range of closureconditions would be less than optimal. Since valve 1036 is utilized forfine control over the circumference of the extruded film tube, it isrelatively important that the valve be operated over its optimal andlinear range of operation. This will allow for better control of theextruded film tube during production operations which follow startup,and which have a significant impact on the product quality produced bythe blown film line and the product quantity produced by the blown filmline. Optimization of the valve will be discussed in greater detailbelow. The process then ends at software block 1151.

[0269] The run mode of operation is depicted in flowchart formcommencing at FIG. 37G. The process commences in software block 1161,and continues at software block 1163, wherein controller 1077 calls theblower balance routine for execution. In broad overview, the controller1077 works to balance the supply and exhaust blowers 1037, 1039 by firstlooking for a recorded value for the operating condition and associatedsupply blower setting from the last time the system was running. Inaccordance with the present invention, a plurality of values for priorproduction runs are stored in memory for use during the run mode ofoperation. An array of such recorded historical run settings is depictedin simplified form in FIG. 37L. As is shown, three columns are recorded,including the master speed potentiometer setting 1301, supply speed1303, and reference volts 1305. For each master speed potentiometersetting available, there is possibly a corresponding recorded historicalvalue of supply speed 1303 and its associated reference voltage 1305.Several dozen to several hundred historical values may be recorded.These values represent prior optimum settings of the supply blower 1037for different operating conditions. Since these particular settings wereused in prior production runs, it is presumed that they weresatisfactory settings. In order to increase the efficiency and accuracyof startup procedures, controller 1077 will first look to historical andrecorded values, if those values exist.

[0270] Returning now to FIG. 37G, the process continues to softwareblock 1165, wherein controller 1077 determines whether a blower balancestartup history exists. In other words, controller 1077 determineswhether there are any prior historical and recorded values for thesetting of supply blower 1037. If not, control passes to software block1167, and the process ends at software block 1169. However, if it isdetermined that a history does exist, control passes to software block1171, wherein controller 1077 examines the blower balancing startuphistory to determine whether there is a value which has been recordedfor the current operating condition as set by the master speedpotentiometer setting. If no particular historical value corresponds tothe current settings, then control passes to software block 1173, andthe process ends at software block 1175. However, if it is determinedthat a prior recorded historical value exists for the operatingcondition of the supply blower 1037, control passes to software block1177, and the process ends at software block 1179.

[0271]FIG. 37H is a flowchart representation of software block 1167 ofFIG. 37G. This routine is executed if a blower balance startup historyexists. The process being at software block 1201, and continues tosoftware block 1203, wherein controller 1077 examines the position ofvalve 1034 (of FIG. 37D). Next, in accordance with software block 1205,controller 1077 determines whether valve 1034 is within its 28-32% stateof closure. As discussed above, this range represents the optimum andlinear operating range of a “bladder” valve which is described herein.If it is determined in software block 1205 by controller 1077 that valve1034 is within its optimum range of operation, the process ends atsoftware block 1211. However, if it is determined in software block 1205that valve 1034 is not within its optimum and linear operating range,the particular percentage of closure is examined to determine whether itfalls above or below the 28-32% range. If the closure state is greaterthan 30%, control passes to software block 1027, wherein the operatingrate of supply blower 1037 is increased by a predetermined amount.Control will then pass back to software block 1205 in order to reexaminethe operating condition of valve 1034. If it is determined at softwareblock 1205 that valve 1034 is below 28% closure, control passes tosoftware block 1209 wherein the rate of operation of supply blower 1037is decreased by a predetermined amount. Control would then return tosoftware block 1205 in order to allow for reexamination of the operatingcondition of valve 1034. This process will repeat until valve 1034 isplaced within its optimum and substantially linear operating state.

[0272]FIG. 371 is a flowchart representation of software block 1173 ofFIG. 37G. This routine corresponds to a situation wherein a blowerbalance startup history does exist, but no recorded and historical valueexists which directly corresponds to the current setting established forthe blown film extrusion line. The process begins at software block1221, and continues at software block 1223, wherein controller 1077fetches the operating speed for the supply blower 1037 from a linearmodel. Function 1220 is a graphical representation of such a linearmodel which maps values of the master speed potentiometer setting tosupply speeds (or the reference voltages which correspond to the supplyspeeds). The model is a simple function (y=mx). The model value whichcorresponds to the current speed potentiometer setting is then appliedto supply blower 1037. Next, in accordance with software block 1225,controller 1077 examines the position of valve 1034 to determine itscurrent state. Then, control passes to software block 1227, whereincontroller 1077 is utilized to determined whether valve 1034 is withinits optimum and substantially linear operating range of 28% to 32% (forthe “bladder” type valve discussed above). If the valve 1034 isoperating within its optimum and substantially linear operating range,control passes to software block 1233, wherein the process ends.However, if it is determined in software block 1227 that valve 1034 isnot within its preferred operating range, the closure state of the valveis examined to determine whether it falls above or below the preferredoperating range. If the closure is greater than 32%, control passes tosoftware block 1229 wherein the operating condition of supply blower1037 is decreased by a non-linear offset component which is depicted byfunction 1222 (in the preferred embodiment, a predetermined constant isadded to the previous function in order to generate a function ofy=mx+b). If it is determined in software block 1229 that the valve isoperating below the 28% closure condition, control passes to softwareblock 1231, wherein the operation of supply blower 1037 is increased bya non-linear offset component (in this situation, and in the preferredembodiment of the present invention, a constant term is added to theprevious function in order to utilize a function of y=mx−b). Thisprocess is repeated until the valve 1034 is placed in its optimum rangeof operation.

[0273]FIG. 37J is a flowchart representation of software block 1177 ofFIG. 37G. In this situation, controller 1077 has determined that ablower balance startup history does exist, and that there is a value inthe historical log which directly corresponds to the current masterspeed potentiometer setting. The process begins at software block 1241and continues at software block 1243, wherein controller 1077 utilizesthe last recorded value for the operating condition of supply blower1037. Then, in accordance with software block 1245, the controllerdetermines whether the extruded film tube (or “bubble”) is at its propersize. Next, in accordance with software block 1247, controller 1077determines whether the recorded value which is utilized for establishingthe setting of supply blower 1037 places valve 1034 within its optimumrange of operation (which, in the preferred embodiment for “bladder”type valves, is 28% to 32%). If it is determined in software 1247 thatvalve 1034 is not operating in its preferred range of positions, controlpasses to software block 1249, and the process ends at software block1251. However, if it is determined in software block 1247 that the valveis indeed operating within its preferred range of positions, controlpasses to software block 1253, wherein the balance mode is not entered,and the process ends at software block 1255.

[0274]FIG. 37K is a flowchart representation of software block 1249 ofFIG. 37J, and describes the balance mode of operation in accordance withthe preferred embodiment of the present invention. The balance mode ofoperation is entered if the historical recorded value for the setting ofsupply blower 1037 does not place the valve in its preferred range ofoperation. The purpose of the balance mode of operation is to allow theoperator to obtain direct control over the operating condition of supplyblower 1034. In the balance mode of operation, the position of valve 134is locked to 30%. In control panel 1079, a ratio is displayed whichrepresents the ratio of the running speeds of the supply blower 1037 andthe exhaust blower 1039. An indication of 50% means that both blowersare running at the same speed. An indication greater than 50% means thatthe supply blower is running faster than the exhaust blower (which isthe normal condition). An indication of less than 50% means that thesupply blower is running slower than the exhaust blower. The operatorcan manually adjust the balance by using buttons 1097, 1092 (of FIG.37D). Selecting the negative button will cause the supply blower to slowdown.

[0275] With reference to FIG. 37K, the process commences at softwareblock 1261 and continues at software block 1263, wherein controller 1077locks the valve position to 30%. Next, in accordance with software block1265, control panel 1079 is utilized to display the “relative ratio”number. Then, in accordance with software block 1067, controller 1077monitors for operator input through depression of either the negativebutton 1090 or the positive button 1092. In software block 1269,controller 1077 monitors for selection of the negative button. If thenegative button is selected, control passes to software block 1271,wherein the supply blower is slowed down. In accordance with softwareblock 1273, controller 1077 monitors for selection of the positivebutton. If the positive button is selected, control passes to softwareblock 1275, wherein the supply blower is speeded up. In accordance withsoftware block 1277, controller 1077 monitors for selection of aproduction mode of operation by the operator. If the production mode isselected, in accordance with software block 1279, controller 1077records the setting in memory (in the table of FIG. 37L) and the processends at software block 1281.

[0276]FIG. 37M is a flowchart representation of a bubble break routinewhich is utilized after the initial steps of startup have been concludedin order to detect bubble break or collapse, sound an alarm, andoptionally shut down the blown film line. The bubble break detectionroutine is suppressed during early phases of the startup in order toallow the operator to get the bubble started. The process commences atsoftware block 1321, wherein the bubble break routine is called forexecution. Next, in accordance with software block 1323, controller 1077determines whether the blown film line is operational, and the exhaustblower is in an on condition. In this way, the bubble break routine issuppressed until the operator manually activates the exhaust blower. Inaccordance with software block 1325, controller 1077 starts a timerdelay (which is operator-configurable in the range of 1-10 minutes)which allows an amount of time sufficient for the operator to get theblown film line started. In accordance with software block 1327,controller 1077 monitors the bubble position sensor in order todetermine the location of the bubble. In accordance with software block1329, the bubble position sensor is monitored to determine whether thereis a loss of signal. If no signal loss occurs, control returns tosoftware block 1327. However, if the position sensor signal is lost,control passes to software block 1331, wherein a software timer isinitiated. Then, in accordance with software block 1333, controller 1077determines whether the signal is still gone. If not, control passes tosoftware block 1327. If so, control passes to software block 1339,wherein controller 1077 determines whether the second software timer has“timed out”. If not, control returns to software block 1333; if so,control passes to software block 1337, where an alarm is sounded.Alternatively, and concurrently with the sounding of the alarm, theblown film line may be disabled. The routine ends at software block1339. In accordance with the present invention, controller 1077 isutilized to continuously monitor the condition of the exhaust blowerthroughout the entire process. Any change in condition of the exhaustblower will automatically reset the bubble break detection routine toits initial condition. In this manner, the bubble break routine willonly run after the operator has been provided with a sufficient time inwhich to get the extruded film tube within the nips, but only becomesoperational if the exhaust blower has been activated. Once the positionsignal has been lost for a sufficiently long time interval, the bubblebreak detector will at least sound an alarm in order to warn of likelybreak or collapse of the bubble. Since the system automatically resetsitself upon any change in condition of the exhaust blower, it willbecome initialized for the next startup.

[0277] Although the invention has been described with reference to aspecific embodiment, this description is not meant to be construed in alimiting sense. Various modifications of the disclosed embodiment aswell as alternative embodiments of the invention will become apparent topersons skilled in the art upon reference to the description of theinvention. It is therefore contemplated that the appended claims willcover any such modifications or embodiments that fall within the truescope of the invention.

What is claimed is:
 1. In a blown film extrusion apparatus in which filmis extruded as a tube from an annular die and then pulled along apredetermined path, an apparatus for startup of said extruded film tube,comprising: (a) means for varying a quantity of air within said extrudedfilm tube, including: (1) a supply blower which supplies air to saidextruded film tube in an amount corresponding to a supply controlsignal, and (2) an exhaust blower which exhausts air from said extrudedfilm tube in an amount corresponding to an exhaust control signal; and(b) a controller member including executable program instructions whichdefine at least one control routine for automatic and coordinatedcontrol of said means for varying during starting of said extruded filmtube by directing a series of supply control signals to said supplyblower and exhaust control signals to said exhaust blower.
 2. Anapparatus according to claim 1, further comprising: (c) a controlinterface for receiving operator instructions during startup of saidextruded film tube; and (d) wherein said controller further includesprogram instructions for receiving said operator instructions andintegrating said operator instructions into said at least one controlroutine.
 3. An apparatus according to claim 1, wherein said at leastcontrol routine comprises at least one of the following routines: (a) astartup routine wherein said controller member initiates operation ofsaid supply blower and said exhaust blower by first initiating operationof said supply blower in accordance with at least one predeterminedoperating parameter, and then initiating said exhaust blower inaccordance with at least one predetermined operating parameter; and (b)a blower optimization routine wherein at least one of (1) said supplycontrol signal, and (2) said exhaust control signal is determined atleast in part from at least one prior recorded control signal.
 4. Anapparatus according to claim 3, wherein said startup routine includesexecutable program instructions for: (1) initially increasing airsupplied by said supply blower to said extruded film tube in accordancewith a predetermined ramping function until said extruded film tube issubstantially closed; and (2) then increasing air exhausted by saidexhaust blower from said extruded film tube in accordance with apredetermined ramping function.
 5. An apparatus according to claim 4,wherein said startup routine further includes executable programinstructions for: (3) continued increasing operation of at least one ofsaid supply blower and said exhaust blower in accordance with at leastone predetermined function.
 6. An apparatus according to claim 1,further comprising: (c) a valve member, under control of said controllermember, for varying admission of air into said extruded film tube andfor controlling the circumference of said extruded film tube afterstartup of said extruded film tube.
 7. An apparatus according to claim6, wherein said at least one control routine comprises at least one ofthe following routines: (a) a startup routine wherein said controllermember initiates operation of said supply blower and said exhaust blowerby first initiating operation of said supply blower in accordance withat least one predetermined operating parameter, and then initiating saidexhaust blower in accordance with at least one predetermined operatingparameter; and (b) a blower optimization routine wherein at least one of(1) said supply control signal, and (2) said exhaust control signal isdetermined at least in part from at least one prior recorded controlsignal; and (c) a valve optimization routine wherein an operatingcondition is established for at least one of (1) said supply blower, and(2) said exhaust blower in a manner which optimizes operation of saidvalve member.
 8. An apparatus according to claim 7, wherein, during saidvalve optimization routine operating conditions are established for atleast one of (1) said supply blower, and (2) said exhaust blower, inorder to allow said valve member to operate in a preferred andsubstantially linear range of closure conditions.
 9. An apparatusaccording to claim 1, further comprising: (c) at least one transducerfor producing a signal corresponding to a detected position of saidextruded film tube; (d) wherein said at least one control routineincludes: (1) a bubble break detection routine wherein said signalgenerated by said at least one transducer is utilized in combinationwith at least one software timer in order to detect a break in saidextruded film tube.
 10. An apparatus for extruding a film tube,comprising: (a) a die member; (b) means for supplying molten film tosaid die member; (c) a supply blower for supplying air to said extrudedfilm tube in an amount corresponding to a supply control signal; (d) anexhaust blower for exhausting air from said extruded film tube in anamount corresponding to an exhaust control signal; (e) a valve forcontrolling air flow from said supply blower to said die member inresponse to a valve control signal; (f) a position sensor for providinga signal indicative of the size of said extruded film tube; (g) Acontroller member including executable instructions which define atleast one control routine; (h) a control interface for receivingoperator instructions; and (i) said at least one control routineincluding: a startup routine for automatic and coordinated control ofsaid supply blower and said exhaust blower during startup of saidextruded film tube.
 11. An apparatus according to claim 10, wherein saidat least one control routine includes: (1) a startup routine whereinsaid controller member initiates operation of said supply blower andsaid exhaust blower by first initiating operation of said supply blowerin accordance with at least one predetermined operating parameter, andthen initiating operation of said exhaust blower in accordance with atleast one predetermined operating parameter: and (2) a bloweroptimization routine wherein at least one of (a) said supply controlsignal, and (b) said exhaust control signal is determined at least inpart from at least one prior recorded control signal.
 12. An apparatusaccording to claim 11, wherein said startup routine includes executableprogram instructions for: (a) initially increasing air supplied by saidsupply blower to said extruded film tube in accordance with apredetermined ramping function until said extruded film tube issubstantially closed; and (b) then increasing air exhausted by saidexhaust blower from said extruded film tube in accordance with apredetermined ramping function.
 13. An apparatus according to claim 12,wherein said startup routine includes executable program instructionsfor: (e) continued increasing operation of at least one of said supplyblower and said exhaust blower in accordance with at least onepredetermined function.
 14. An apparatus according to claim 11, whereinsaid at least one control routine her includes: (3) a valve optimizationroutine wherein an operating condition is established for at least oneof (a) said supply blower, and (b) said exhaust blower in a manner whichoptimizes operation of said valve member.
 15. An apparatus according toclaim 14 wherein, during said valve optimization routine, operatingconditions are established for at least one of (1) said supply blowerand (2) said exhaust blower, in order to allow said valve member tooperate in a preferred and substantially linear range of closureconditions.
 16. An apparatus according to claim 11, wherein said atleast one control routine further includes: (3) a bubble break detectionroutine wherein said signal generated by said position sensor isutilized in combination with at least one software timer in order todetect a break in said extruded film tube.
 17. A method of startup of anextruded film tube in a blown film extrusion apparatus, comprising: (a)providing a controller, a supply blower, and an exhaust blower; (b)utilizing said supply blower to supply air to said extruded film tube inan amount corresponding to a supply control signal; (c) utilizing saidexhaust blower to exhaust air from said extruded film tube in an amountcorresponding to an exhaust control signal; and (d) utilizing saidcontroller member for executing program instructions which define atleast one control routine for automatic and coordinated control duringstarting of said extruded film tube by directing a series of supplycontrol signals to said supply blower and exhaust control signals tosaid exhaust blower.
 18. A method according to claim 17, furthercomprising: (e) providing a control interface for receiving operatorinstructions during startup of said extruded film tube; and (f) whereinsaid controller further executes program instructions for receiving saidoperator instructions and integrating said operator instructions intosaid at least one control routine.
 19. A method according to claim 17,further comprising: (e) utilizing said controller to execute programinstructions of a startup routine wherein said controller memberinitiates operation of said supply blower and said exhaust blower byfirst initiating operation of said supply blower in accordance with atleast one predetermined operating parameter, and then initiating saidexhaust blower in accordance with at least one predetermined operatingparameter; and (f) utilizing said controller to execute programinstructions of a blower optimization routine wherein at least one of(1) said supply control signal, and (2) said exhaust control signal isdetermined at least in part from at least one prior recorded controlsignal.
 20. A method according to claim 19, wherein said startup routineincludes executable program instructions for: (1) initially increasingair supplied by said supply blower to said extruded film tube inaccordance with a predetermined ramping function until said extrudedfilm tube is substantially closed; and (2) then increasing air exhaustedby said exhaust blower from said extruded film tube in accordance with apredetermined ramping function.
 21. A method according to claim 20,wherein said startup routine further includes executable programinstructions for: (3) continued increasing operation of at least one ofsaid supply blower and said exhaust blower in accordance with at leastone predetermined function.
 22. A method according to claim 17, furthercomprising: (e) providing a valve member, under control of saidcontroller member, for varying admission of air into said extruded filmtube and for controlling the circumference of said extruded film tubeafter startup of said extruded film tube.
 23. A method according toclaim 22, wherein said at least one control routine comprises at leastone of the following routines: (1) a startup routine wherein saidcontroller member initiates operation of said supply blower and saidexhaust blower by first initiating operation of said supply blower inaccordance with at least one predetermined operating parameter, and theninitiating said exhaust blower in accordance with at least onepredetermined operating parameter; and (2) a blower optimization routinewherein at least one of (1) said supply control signal, and (2) saidexhaust control signal is determined at least in part from at least oneprior recorded control signal; and (3) a valve optimization routinewherein an operating condition is established for at least one of (1)said supply blower, and (2) said exhaust blower in a manner whichoptimizes operation of said valve member.
 24. A method according toclaim 23, wherein, during said valve optimization routine, operatingconditions are established for at least one of (1) said supply blower,and (2) said exhaust blower, in order to allow said valve member tooperate in a preferred and substantially linear range of closureconditions.
 25. A method according to claim 17, further comprising: (e)providing at least one transducer for producing a signal correspondingto a detected position of said extruded film tube; (f) wherein said atleast one control routine includes a bubble break detection routinewherein said signal generated by said at least one transducer isutilized in combination with at least one software timer in order todetect a break in said extruded film tube.
 26. An improved blown filmextrusion apparatus, comprising: (a) a die for receiving molten materialand extruding a film tube; (b) a controller member; (c) a supply blowerwhich is responsive to command signals from said controller forsupplying a variable quantity of air to said film tube; (d) an airflowpath between said supply blower and said die; (e) an exhaust blowerwhich is responsive to command signals from said controller forexhausting a variable quantity of air from said film tube; (f) an airflow control member which is at least in-part responsive to commandsignals from said controller member for varying a quantity of airpassing within said air flow path, and which includes: (1) a housingwith an inlet, an outlet, and an air path defined therethrough; (2) atleast one selectively-expandable flow restriction member disposed insaid housing in said air flow path; and (3) wherein said air flow memberselectively expands and reduces said at least one selectively-expandableflow restriction member to moderate air flow through said air flow path.(g) at least one program routine executable by said controller memberwhich optimizes operation of said supply blower, said exhaust blower,and said air flow control member.
 27. An improved blown film apparatusaccording to claim 26: (h) wherein said at least oneselectively-expandable flow restriction member includes a bladder memberwhich selectively communicates with a control fluid; and (i) whereinapplication of said control fluid to said at least oneselectively-expandable flow restriction member causes expansion andreduction of said at least one selectively-expandable flow restrictionmember.
 28. An improved blown film apparatus according to claim 26,wherein said air flow control member includes: (1) a plurality ofhousings, each having an inlet, outlet, and an air flow path definedtherethrough; (2) a plurality of selectively-expandable flow restrictionmembers disposed in each of said housings; and (3) with each flow paththrough said plurality of housings in at least one of (a) series, and(b) parallel communication with said selected others of said air flowpaths.
 29. An improved blown film apparatus, according to claim 26,wherein: (h) expansion of said at least one selectively-expandable flowrestriction member restricts said air path defined through said housing;and (i) reduction of said at least one selectively-expandable flowrestriction member expands said air path defined through said housing.